fix: detect PlainDate by calendarId in resolveFirstWeekSpec

package/src/resolve-first-week.test.ts

@@ -61,6 +61,16 @@ describe("resolveFirstWeekSpec", () => {
     const result = resolveFirstWeekSpec(spec, weekStartDay, T);
     expect(result.toString()).toBe(expected);
   });
+
+  it("converts a non-ISO-calendar PlainDate to ISO before snapping", () => {
+    // Buddhist 2026-03-15 (a Sunday) — its calendar fields read year 2569.
+    // Pre-fix the PlainDate fell through to the {month,year,day} branch and
+    // those fields were reinterpreted as ISO → ~543 years off.
+    const buddhist = T.PlainDate.from("2026-03-15").withCalendar("buddhist");
+    const result = resolveFirstWeekSpec(buddhist, 0, T);
+    expect(result.toString()).toBe("2026-03-15");
+    expect(result.calendarId).toBe("iso8601");
+  });
 });
 
 describe("resolveFirstWeek", () => {

package/src/resolve-first-week.ts

@@ -40,9 +40,15 @@ export function resolveFirstWeekSpec(
   T: TemporalNamespace,
   timeZone?: string,
 ): Temporal.PlainDate {
-  // 1. PlainDate — Temporal objects have a "calendar" property
-  if ("calendar" in spec) {
-    return snapToWeekStart(spec as Temporal.PlainDate, weekStartDay);
+  // 1. Temporal.PlainDate — identified by its `calendarId` (modern Temporal
+  //    and @js-temporal/polyfill 0.5+ expose `calendarId`, not `calendar`).
+  //    Convert non-ISO calendars to ISO so the rest of the pipeline — which
+  //    is ISO-only — stays consistent; the week snap itself is
+  //    calendar-independent.
+  if ("calendarId" in spec) {
+    const pd = spec as Temporal.PlainDate;
+    const iso = pd.calendarId === "iso8601" ? pd : pd.withCalendar("iso8601");
+    return snapToWeekStart(iso, weekStartDay);
   }
 
   // 2. Native Date