feat: writable WASM sources (Phase C2)

[adnaan]

Context

This is the final item from the progressive complexity plan. WASM sources currently implement Source (read-only). Extending them to implement WritableSource would let users build fully custom CRUD backends in TinyGo.

Progressive complexity tiers:

• Tier 0: Pure markdown (auto-tasks) ✅
• Tier 1: Markdown + YAML (auto-tables) ✅ PR feat: auto-tables — infer interactive UI from markdown tables + YAML sources #207
• Tier 2: HTML + lvt-* attributes ✅ (existed)
• Tier 3: Go templates ✅ (existed)
• Tier 4: Custom sources — WASM read-only ✅, writable WASM ← this issue

Task Description

Extend internal/wasm/source.go (WasmSource) to implement the WritableSource interface:

type WritableSource interface {
    Source
    WriteItem(ctx context.Context, action string, data map[string]interface{}) error
    IsReadonly() bool
}

The WASM module would need to export a WriteItem(action, data) function (or similar) that tinkerdown calls for Add/Update/Delete operations. The TinyGo WASM SDK would need a corresponding update.

Implementation notes

• WasmSource already has IsReadonly() (returns true) — change to check if the module exports a write function
• Add WriteItem that calls the WASM module's exported write function
• Update the wasm-source template (cmd/tinkerdown/commands/templates/wasm-source/) to show a writable example
• Update docs/sources/wasm.md

Files to modify

• internal/wasm/source.go — implement WritableSource
• cmd/tinkerdown/commands/templates/wasm-source/ — update template
• docs/sources/wasm.md — document writable WASM

---

From the progressive complexity plan (Phase C2).

[adnaan]

Progress (M5 Phase 3, #310) — CLI parity landed; keeping this open

tinkerdown cli now constructs wasm sources (createCLISource previously fell through to "unsupported source type for CLI"), so a WASM source has the same CRUD surface on the CLI as under serve. Also fixed a load-breaking bug found while writing the docs: docs/sources/wasm.md and the wasm-source template used a non-existent module: key (the real key is path:) and config: (real: options:), so following the docs failed to load. A Read-only vs. writable section now documents the write-export contract + CLI parity.

Verified running: tinkerdown cli … list quotes returns rows; tinkerdown cli … add against a read-only module returns Error: source is read-only; a unit test covers construct + Fetch + IsReadonly + the WriteItem guard error.

Why this stays open: the feature this issue names — a successful write through a write-exporting module — still has zero verified coverage in any transport (CLI or WebSocket). "Verify, don't infer" found the assumption that examples/lvt-source-wasm-test/ is a writable path to be wrong: quotes.wasm is read-only (no write export), it's the only .wasm in the repo, and there's no TinyGo in this environment to build a write-exporting fixture (a committed one that can't be rebuilt would be the #295/#297 provenance trap). Tracked as #311 — a reproducible TinyGo fixture wired into make + a write-landing test over CLI and WS. This issue closes when that write path is demonstrated end-to-end.

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https://claude.ai/code/session_018M9pJSPmG6i1D8s6rpEV4h