diff --git a/.changeset/rate-basis-round-trip-utils.md b/.changeset/rate-basis-round-trip-utils.md
new file mode 100644
index 000000000..6c3b82bd6
--- /dev/null
+++ b/.changeset/rate-basis-round-trip-utils.md
@@ -0,0 +1,5 @@
+---
+'@app/ratewise': patch
+---
+
+新增來回換匯成本與計價基準判定的內部工具模組，供後續在幣別頁揭露「換出去再換回來」的實際損失與交叉換算警示。成本計算限定台銀牌告的報價單位，換錢所的反向報價會被單位保護擋下而不產生錯誤數字。本次尚未有使用者可見變更。
diff --git a/apps/ratewise/src/i18n/locales/en.ts b/apps/ratewise/src/i18n/locales/en.ts
index 03e0a0e68..60e864c14 100644
--- a/apps/ratewise/src/i18n/locales/en.ts
+++ b/apps/ratewise/src/i18n/locales/en.ts
@@ -85,6 +85,12 @@ const en = {
     cashSellBasis: 'Cash sell rate trend',
   },
 
+  // Rate Basis（計價基準揭露；僅在 selector 表達不出來時顯示）
+  rateBasis: {
+    mid: 'Mid-market rate (not a dealable quote)',
+    cross: 'Cross conversion (via TWD, two spreads applied)',
+  },
+
   // Multi Currency Converter
   multiConverter: {
     title: 'Multi-Currency',
diff --git a/apps/ratewise/src/i18n/locales/ja.ts b/apps/ratewise/src/i18n/locales/ja.ts
index 3d9eb7613..de50aa996 100644
--- a/apps/ratewise/src/i18n/locales/ja.ts
+++ b/apps/ratewise/src/i18n/locales/ja.ts
@@ -85,6 +85,12 @@ const ja = {
     cashSellBasis: '現金売レートの推移',
   },
 
+  // Rate Basis（計價基準揭露；僅在 selector 表達不出來時顯示）
+  rateBasis: {
+    mid: '仲値（取引レートではありません）',
+    cross: 'クロス換算（TWD 経由、スプレッド 2 回分）',
+  },
+
   // Multi Currency Converter
   multiConverter: {
     title: '複数通貨換算',
diff --git a/apps/ratewise/src/i18n/locales/ko.ts b/apps/ratewise/src/i18n/locales/ko.ts
index 3f698830f..9f0d24ecd 100644
--- a/apps/ratewise/src/i18n/locales/ko.ts
+++ b/apps/ratewise/src/i18n/locales/ko.ts
@@ -85,6 +85,12 @@ const ko = {
     cashSellBasis: '현금 매도율 추이',
   },
 
+  // Rate Basis（計價基準揭露；僅在 selector 表達不出來時顯示）
+  rateBasis: {
+    mid: '중간 환율 (거래 가능 호가 아님)',
+    cross: '교차 환산 (TWD 경유, 스프레드 2회 적용)',
+  },
+
   // Multi Currency Converter
   multiConverter: {
     title: '복수 통화 환산',
diff --git a/apps/ratewise/src/i18n/locales/zh-TW.ts b/apps/ratewise/src/i18n/locales/zh-TW.ts
index 9c07b9392..571ce5788 100644
--- a/apps/ratewise/src/i18n/locales/zh-TW.ts
+++ b/apps/ratewise/src/i18n/locales/zh-TW.ts
@@ -85,6 +85,12 @@ const zhTW = {
     cashSellBasis: '現金賣出走勢',
   },
 
+  // Rate Basis（計價基準揭露；僅在 selector 表達不出來時顯示）
+  rateBasis: {
+    mid: '參考中間價（非交易報價）',
+    cross: '交叉換算（經台幣中轉，計入兩次價差）',
+  },
+
   // Multi Currency Converter
   multiConverter: {
     title: '多幣別換算',
diff --git a/apps/ratewise/src/utils/__tests__/rateBasisLabel.test.ts b/apps/ratewise/src/utils/__tests__/rateBasisLabel.test.ts
new file mode 100644
index 000000000..f4046b020
--- /dev/null
+++ b/apps/ratewise/src/utils/__tests__/rateBasisLabel.test.ts
@@ -0,0 +1,92 @@
+import { describe, it, expect } from 'vitest';
+import {
+  getRateBasisKind,
+  getRateBasisLabelKey,
+  shouldDiscloseRateBasis,
+  type RateBasisKind,
+} from '../rateBasisLabel';
+import zhTW from '../../i18n/locales/zh-TW';
+import en from '../../i18n/locales/en';
+import ja from '../../i18n/locales/ja';
+import ko from '../../i18n/locales/ko';
+
+describe('getRateBasisKind', () => {
+  it('rateMode 為 mid 時一律回傳 mid（優先於 cross）', () => {
+    expect(getRateBasisKind('USD', 'JPY', 'mid')).toBe('mid');
+    expect(getRateBasisKind('TWD', 'USD', 'mid')).toBe('mid');
+    expect(getRateBasisKind('USD', 'USD', 'mid')).toBe('mid');
+  });
+
+  it('兩端皆非台幣時為 cross（經台幣中轉吃兩次價差）', () => {
+    expect(getRateBasisKind('USD', 'JPY', 'auto')).toBe('cross');
+    expect(getRateBasisKind('JPY', 'KRW', 'sell')).toBe('cross');
+  });
+
+  it('任一端為台幣時為 direct（方向已由 selector 表達）', () => {
+    expect(getRateBasisKind('TWD', 'USD', 'auto')).toBe('direct');
+    expect(getRateBasisKind('USD', 'TWD', 'auto')).toBe('direct');
+    expect(getRateBasisKind('TWD', 'JPY', 'sell')).toBe('direct');
+  });
+
+  it('同幣別不構成換匯，為 direct', () => {
+    expect(getRateBasisKind('USD', 'USD', 'auto')).toBe('direct');
+    expect(getRateBasisKind('TWD', 'TWD', 'sell')).toBe('direct');
+  });
+});
+
+describe('getRateBasisLabelKey', () => {
+  it('mid 與 cross 回傳對應的 i18n key', () => {
+    expect(getRateBasisLabelKey('mid')).toBe('rateBasis.mid');
+    expect(getRateBasisLabelKey('cross')).toBe('rateBasis.cross');
+  });
+
+  it('direct 不需標籤故回傳 null', () => {
+    expect(getRateBasisLabelKey('direct')).toBeNull();
+  });
+});
+
+describe('shouldDiscloseRateBasis', () => {
+  it('僅 mid 與 cross 需要揭露', () => {
+    expect(shouldDiscloseRateBasis('mid')).toBe(true);
+    expect(shouldDiscloseRateBasis('cross')).toBe(true);
+    expect(shouldDiscloseRateBasis('direct')).toBe(false);
+  });
+});
+
+describe('i18n key 完整性', () => {
+  const locales = { zhTW, en, ja, ko };
+  const disclosedKinds: RateBasisKind[] = ['mid', 'cross'];
+
+  it('四語系皆有 rateBasis 區塊', () => {
+    for (const [name, locale] of Object.entries(locales)) {
+      expect(locale.rateBasis, `${name} 缺少 rateBasis 區塊`).toBeDefined();
+    }
+  });
+
+  it('四語系皆備齊需揭露的 kind 且非空字串', () => {
+    for (const [name, locale] of Object.entries(locales)) {
+      for (const kind of disclosedKinds) {
+        const value = (locale.rateBasis as Record<string, string | undefined>)[kind];
+        expect(value, `${name}.rateBasis.${kind} 缺失`).toBeTruthy();
+        expect(value?.trim().length ?? 0, `${name}.rateBasis.${kind} 為空`).toBeGreaterThan(0);
+      }
+    }
+  });
+
+  it('不得為 direct 建立文案（該狀態不顯示標籤）', () => {
+    for (const [name, locale] of Object.entries(locales)) {
+      expect(
+        (locale.rateBasis as Record<string, string | undefined>)['direct'],
+        `${name} 不應有 rateBasis.direct`,
+      ).toBeUndefined();
+    }
+  });
+
+  it('zh-TW 文案明示中間價非可成交報價', () => {
+    expect(zhTW.rateBasis.mid).toContain('非交易報價');
+  });
+
+  it('zh-TW 交叉換算文案明示計入兩次價差', () => {
+    expect(zhTW.rateBasis.cross).toContain('兩次價差');
+  });
+});
diff --git a/apps/ratewise/src/utils/__tests__/roundTripCost.test.ts b/apps/ratewise/src/utils/__tests__/roundTripCost.test.ts
new file mode 100644
index 000000000..7cba59c8c
--- /dev/null
+++ b/apps/ratewise/src/utils/__tests__/roundTripCost.test.ts
@@ -0,0 +1,145 @@
+import { describe, it, expect } from 'vitest';
+import {
+  computeRoundTripCost,
+  computeRoundTripLossTwd,
+  classifyRoundTripSeverity,
+  deriveCashBuy,
+  ROUND_TRIP_SEVERITY_THRESHOLDS,
+} from '../roundTripCost';
+import { SEO_RATE_EXAMPLES } from '../../config/generated/seo-rate-examples';
+
+describe('computeRoundTripCost', () => {
+  it('以 (sell - buy) / sell 計算損失率', () => {
+    // 取 USD 量級的合成值：賣出 32.56、買入 31.89，價差 0.67
+    const cost = computeRoundTripCost(32.56, 31.89);
+    expect(cost).not.toBeNull();
+    expect(cost!.lossPct).toBe(2.06);
+    expect(cost!.spread).toBeCloseTo(0.67, 6);
+  });
+
+  it('損失率與換匯金額無關（僅由價差決定）', () => {
+    const a = computeRoundTripCost(100, 90);
+    const b = computeRoundTripCost(0.001, 0.0009);
+    expect(a!.lossPct).toBe(b!.lossPct);
+  });
+
+  it('缺任一報價時回傳 null', () => {
+    expect(computeRoundTripCost(32.65, null)).toBeNull();
+    expect(computeRoundTripCost(null, 31.98)).toBeNull();
+    expect(computeRoundTripCost(undefined, undefined)).toBeNull();
+  });
+
+  it('非有限數回傳 null', () => {
+    expect(computeRoundTripCost(NaN, 31.98)).toBeNull();
+    expect(computeRoundTripCost(32.65, Infinity)).toBeNull();
+  });
+
+  it('sell <= 0 或 buy <= 0 回傳 null（避免除以零）', () => {
+    expect(computeRoundTripCost(0, 0)).toBeNull();
+    expect(computeRoundTripCost(-1, -2)).toBeNull();
+  });
+
+  it('買入高於賣出代表傳入反向報價單位，拒絕計算', () => {
+    expect(computeRoundTripCost(30, 31)).toBeNull();
+  });
+
+  it('換錢所 KRW_PER_TWD 報價（sell 46.0 / buy 46.7）回傳 null 而非錯誤數字', () => {
+    // 明洞換錢所報價單位為每 1 台幣換得 N 韓元，方向與台銀 TWD_PER_FOREIGN 相反；
+    // 此處刻意驗證單位保護生效，避免對換錢所資料算出反號的損失率。
+    expect(computeRoundTripCost(46.0, 46.7)).toBeNull();
+  });
+
+  it('買賣同價時損失率為 0', () => {
+    const cost = computeRoundTripCost(30, 30);
+    expect(cost!.lossPct).toBe(0);
+    expect(cost!.severity).toBe('low');
+  });
+});
+
+describe('classifyRoundTripSeverity', () => {
+  it('依門檻分級，邊界值歸入較嚴重的一級', () => {
+    expect(classifyRoundTripSeverity(0)).toBe('low');
+    expect(classifyRoundTripSeverity(4.99)).toBe('low');
+    expect(classifyRoundTripSeverity(ROUND_TRIP_SEVERITY_THRESHOLDS.medium)).toBe('medium');
+    expect(classifyRoundTripSeverity(14.99)).toBe('medium');
+    expect(classifyRoundTripSeverity(ROUND_TRIP_SEVERITY_THRESHOLDS.high)).toBe('high');
+    expect(classifyRoundTripSeverity(32.11)).toBe('high');
+  });
+});
+
+describe('computeRoundTripLossTwd', () => {
+  it('換 30000 台幣的 USD 再換回，損失約 618 元', () => {
+    const cost = computeRoundTripCost(32.56, 31.89);
+    expect(computeRoundTripLossTwd(30000, cost)).toBe(618);
+  });
+
+  it('cost 為 null 時回傳 null', () => {
+    expect(computeRoundTripLossTwd(30000, null)).toBeNull();
+  });
+
+  it('金額為負或非有限數時回傳 null', () => {
+    const cost = computeRoundTripCost(32.56, 31.89);
+    expect(computeRoundTripLossTwd(-1, cost)).toBeNull();
+    expect(computeRoundTripLossTwd(NaN, cost)).toBeNull();
+  });
+
+  it('金額為 0 時損失為 0', () => {
+    const cost = computeRoundTripCost(32.56, 31.89);
+    expect(computeRoundTripLossTwd(0, cost)).toBe(0);
+  });
+});
+
+describe('deriveCashBuy', () => {
+  it('自 bankMid 代數還原買入價（非估計，無誤差）', () => {
+    // bankMid = (cashBuy + cashSell) / 2 → cashBuy = 2 * bankMid - cashSell
+    expect(deriveCashBuy(32.225, 32.56)).toBeCloseTo(31.89, 6);
+  });
+
+  it('bankMid 為 null 代表無現金買入報價，回傳 null', () => {
+    expect(deriveCashBuy(null, 32.56)).toBeNull();
+    expect(deriveCashBuy(undefined, 32.56)).toBeNull();
+  });
+
+  it('非有限數或推導出非正值時回傳 null', () => {
+    expect(deriveCashBuy(NaN, 32.56)).toBeNull();
+    expect(deriveCashBuy(10, 30)).toBeNull();
+  });
+});
+
+describe('真實牌告資料迴歸', () => {
+  const entries = Object.entries(SEO_RATE_EXAMPLES);
+
+  it('所有具 bankMid 的幣別皆可還原買入價並算出來回成本', () => {
+    const withMid = entries.filter(([, ex]) => ex.bankMid != null);
+    expect(withMid.length).toBeGreaterThan(0);
+    for (const [code, ex] of withMid) {
+      const buy = deriveCashBuy(ex.bankMid, ex.cashSell);
+      expect(buy, `${code} 應可還原買入價`).not.toBeNull();
+      const cost = computeRoundTripCost(ex.cashSell, buy);
+      expect(cost, `${code} 應可算出來回成本`).not.toBeNull();
+      expect(cost!.lossPct).toBeGreaterThanOrEqual(0);
+    }
+  });
+
+  it('還原的買入價恆低於賣出價（價差方向正確）', () => {
+    for (const [code, ex] of entries) {
+      const buy = deriveCashBuy(ex.bankMid, ex.cashSell);
+      if (buy != null) {
+        expect(buy, `${code} 買入不應高於賣出`).toBeLessThan(ex.cashSell);
+      }
+    }
+  });
+
+  it('東南亞現鈔幣別的來回損失顯著高於主要貨幣', () => {
+    const costOf = (code: 'USD' | 'IDR') => {
+      const ex = SEO_RATE_EXAMPLES[code];
+      expect(ex, `${code} 應存在於 SEO_RATE_EXAMPLES`).toBeDefined();
+      return computeRoundTripCost(ex!.cashSell, deriveCashBuy(ex!.bankMid, ex!.cashSell));
+    };
+    const usd = costOf('USD');
+    const idr = costOf('IDR');
+    expect(usd!.severity).toBe('low');
+    expect(idr!.severity).toBe('high');
+    expect(idr!.lossPct).toBeGreaterThan(usd!.lossPct);
+  });
+});
diff --git a/apps/ratewise/src/utils/rateBasisLabel.ts b/apps/ratewise/src/utils/rateBasisLabel.ts
new file mode 100644
index 000000000..a14913bdc
--- /dev/null
+++ b/apps/ratewise/src/utils/rateBasisLabel.ts
@@ -0,0 +1,45 @@
+import type { CurrencyCode, RateMode } from '../features/ratewise/types';
+
+/**
+ * 計價基準語意 SSOT。
+ *
+ * 只揭露 RateTypeSelector 講不出來的事：
+ * - `mid`：中間價非可成交報價，數字本身不代表任何一方向的成交價。
+ * - `cross`：外幣兌外幣需經台幣中轉，買賣價差被吃兩次，畫面上無其他痕跡。
+ * - `direct`：直接買或賣，方向已由 selector 的可見狀態表達，不重複標註。
+ *
+ * 設計原則：恆亮的標籤會退化為視覺壁紙；只在有話要說時出現才具訊號價值。
+ */
+export type RateBasisKind = 'mid' | 'cross' | 'direct';
+
+/** i18n key 前綴；標籤文案集中於 locales 的 rateBasis 區塊。 */
+export const RATE_BASIS_I18N_PREFIX = 'rateBasis' as const;
+
+/** 取得對應的 i18n key；`direct` 不需標籤故回傳 null。 */
+export function getRateBasisLabelKey(kind: RateBasisKind): `rateBasis.${RateBasisKind}` | null {
+  return kind === 'direct' ? null : `${RATE_BASIS_I18N_PREFIX}.${kind}`;
+}
+
+/**
+ * 判定當前換算採用的計價基準。
+ *
+ * 優先序 `mid` > `cross` > `direct`：中間價根本不可成交，
+ * 此警示強於「跨了兩次價差」，兩者同時成立時以 mid 為準。
+ */
+export function getRateBasisKind(
+  fromCurrency: CurrencyCode,
+  toCurrency: CurrencyCode,
+  rateMode: RateMode,
+): RateBasisKind {
+  if (rateMode === 'mid') return 'mid';
+  // 同幣別不構成換匯，無跨價差問題。
+  if (fromCurrency === toCurrency) return 'direct';
+  // 兩端皆非台幣時需經台幣中轉，買賣價差被吃兩次。
+  if (fromCurrency !== 'TWD' && toCurrency !== 'TWD') return 'cross';
+  return 'direct';
+}
+
+/** 該基準是否需要在 UI 顯示標籤。 */
+export function shouldDiscloseRateBasis(kind: RateBasisKind): boolean {
+  return kind !== 'direct';
+}
diff --git a/apps/ratewise/src/utils/roundTripCost.ts b/apps/ratewise/src/utils/roundTripCost.ts
new file mode 100644
index 000000000..9f538720c
--- /dev/null
+++ b/apps/ratewise/src/utils/roundTripCost.ts
@@ -0,0 +1,99 @@
+/**
+ * 來回換匯成本計算。
+ *
+ * 「換出去再換回來」會被銀行賺兩次價差：以賣出價買外幣、以買入價換回台幣。
+ * 損失率僅由買賣價差決定，與換匯金額無關（手續費另計，不在此模組範圍）。
+ *
+ * 推導：持 X 台幣 → 外幣 X/sell → 換回台幣 X*buy/sell
+ *       損失 = X * (sell - buy) / sell
+ */
+
+/** 來回損失嚴重度分級門檻（%）；依 17 幣別實測分布訂定。 */
+export const ROUND_TRIP_SEVERITY_THRESHOLDS = {
+  /** 低於此值為 low */
+  medium: 5,
+  /** 達到此值為 high */
+  high: 15,
+} as const;
+
+export type RoundTripSeverity = 'low' | 'medium' | 'high';
+
+export interface RoundTripCost {
+  /** 來回損失率（%），已四捨五入至小數兩位 */
+  lossPct: number;
+  /** 買賣價差（以台幣計價，每 1 單位外幣） */
+  spread: number;
+  /** 嚴重度分級 */
+  severity: RoundTripSeverity;
+}
+
+/** 依損失率分級；門檻採 [low, medium) / [medium, high) / [high, ∞)。 */
+export function classifyRoundTripSeverity(lossPct: number): RoundTripSeverity {
+  if (lossPct >= ROUND_TRIP_SEVERITY_THRESHOLDS.high) return 'high';
+  if (lossPct >= ROUND_TRIP_SEVERITY_THRESHOLDS.medium) return 'medium';
+  return 'low';
+}
+
+/**
+ * 計算單一牌告類型（現金或即期）的來回成本。
+ *
+ * **報價單位限定 `TWD_PER_FOREIGN`**（每 1 單位外幣 = N 台幣，台銀牌告的形式）。
+ * 換錢所報價為 `KRW_PER_TWD`（每 1 台幣 = N 韓元），該單位下 buy 反而高於 sell
+ * （例如明洞 sell 46.0 / buy 46.7），語意與此函式的參數相反，
+ * 故會落入下方守衛回傳 null——這是刻意的單位保護，不是資料異常。
+ * 換錢所若要計算來回成本，需另行依 `api-semantics-v2` 的 quoteUnit 換算後再呼叫。
+ *
+ * @param sell 賣出價（客戶買外幣所適用），單位 TWD_PER_FOREIGN
+ * @param buy 買入價（客戶換回台幣所適用），單位 TWD_PER_FOREIGN
+ * @returns 缺任一報價、非有限數、sell <= 0，或 buy > sell（單位不符）時回傳 null
+ */
+export function computeRoundTripCost(
+  sell: number | null | undefined,
+  buy: number | null | undefined,
+): RoundTripCost | null {
+  if (sell == null || buy == null) return null;
+  if (!Number.isFinite(sell) || !Number.isFinite(buy)) return null;
+  // sell <= 0 無法作為分母；在 TWD_PER_FOREIGN 單位下 buy 不應高於 sell，
+  // 若成立代表傳入了反向報價單位（如換錢所的 KRW_PER_TWD），拒絕計算以免給出錯誤數字。
+  if (sell <= 0 || buy <= 0 || buy > sell) return null;
+
+  const spread = sell - buy;
+  return {
+    lossPct: Number(((spread / sell) * 100).toFixed(2)),
+    spread: Number(spread.toFixed(6)),
+    severity: classifyRoundTripSeverity((spread / sell) * 100),
+  };
+}
+
+/**
+ * 自台銀自身中間價反推現金買入價。
+ *
+ * `seo-rate-examples` 產出 `bankMid = (cashBuy + cashSell) / 2`，未直接保留 `cashBuy`，
+ * 故買入價需由此還原：`cashBuy = 2 * bankMid - cashSell`。此為代數還原而非估計，
+ * 不引入誤差。`bankMid` 為 null 代表該幣別無現金買入報價。
+ */
+export function deriveCashBuy(
+  bankMid: number | null | undefined,
+  cashSell: number | null | undefined,
+): number | null {
+  if (bankMid == null || cashSell == null) return null;
+  if (!Number.isFinite(bankMid) || !Number.isFinite(cashSell)) return null;
+  const buy = 2 * bankMid - cashSell;
+  return buy > 0 ? Number(buy.toFixed(8)) : null;
+}
+
+/**
+ * 換出再換回的台幣損失金額。
+ *
+ * @param twdAmount 起始台幣金額
+ * @param cost 來回成本
+ * @returns 損失台幣金額（四捨五入至整數）；金額非有限數或為負時回傳 null
+ */
+export function computeRoundTripLossTwd(
+  twdAmount: number,
+  cost: RoundTripCost | null,
+): number | null {
+  if (cost == null) return null;
+  if (!Number.isFinite(twdAmount) || twdAmount < 0) return null;
+  return Math.round((twdAmount * cost.lossPct) / 100);
+}
diff --git a/docs/dev/002_development_reward_penalty_log.md b/docs/dev/002_development_reward_penalty_log.md
index ca2b3dac6..fef31038b 100644
--- a/docs/dev/002_development_reward_penalty_log.md
+++ b/docs/dev/002_development_reward_penalty_log.md
@@ -2,7 +2,7 @@
 
 > 版本：outline-v2-ultra
 > 原則：每筆只保留日期、ID、原因、解法。
-> 本次分數變化：-1（reward 0、penalty 1、neutral 0）｜累計總分：+317
+> 本次分數變化：-1（reward 0、penalty 1、neutral 1）｜累計總分：+316
 
 ## 新增模板（4 行）
 
@@ -13,6 +13,16 @@
 
 ## 條目（新→舊）
 
+- 日期：2026-08-08
+- ID：penalty-stale-branch-analysis-misled-prd-twice
+- 原因：在 checkout 於 feat/single-fold-fluid-fit（落後 main 200+ commits）的主 worktree 上盤點「main 現況」，同一份 PRD 兩度建立在過期程式碼上——先誤判幣別頁 schema 方向有 bug（main 實為自洽），後誤判 RateExample 已有 cashBuy/spotBuy/spotSell（main 僅有 cashSell/bankMid）
+- 解法：撤回 v1.0 主張並改於 origin/main worktree 重驗；新增 deriveCashBuy 以 bankMid 代數還原買入價，刪除 main 無法計算的即期來回損失欄位；往後現況盤點一律在對應 base 的 worktree 執行
+
+- 日期：2026-08-08
+- ID：neutral-ratewise-round-trip-cost-util
+- 原因：換出再換回被銀行賺兩次價差的成本從未量化，17 幣別損失率全距 2.06%～32.11% 的事實無任何模組可產出
+- 解法：新增 roundTripCost 與 rateBasisLabel 純函式模組（含真實牌告迴歸測試與四語系 key 完整性守門），計價基準自 #433 的 8 種精簡為 mid/cross/direct 三種
+
 - 日期：2026-08-08
 - ID：penalty-types-node-major-misjudged-as-patch
 - 原因：盤點 dependabot PR 時只看 mergeStateStatus=CLEAN 與 checks 全綠，未讀版本號就把 @types/node 24→26 判為「patch 級可直接合」，忽略 runtime 鎖在 Node 24（engines/.nvmrc/CI 三處一致）