feat(i18n): 21-language pipeline, landing/docs/API wiring, landing+API translations

Shared Claude Code translation pipeline (scripts/i18n, no API key) plus Astro/VitePress/Scalar i18n wiring. Landing and API reference translated into all 20 languages; docs i18n wiring + English source anchors. The translated docs markdown (apps/docs/<locale>/**, 3,620 files) follows in a companion PR because it exceeds GitHub's per-PR CI file limit.

scripts/i18n/adapter-contract.md

@@ -0,0 +1,26 @@
+<!-- scripts/i18n/adapter-contract.md -->
+# i18n adapter contract
+
+Each surface (landing UI, landing SEO, docs, API spec) implements one adapter object:
+
+```js
+export const adapter = {
+  name: "docs",                                  // unique surface key used on the CLI
+  async extract() { /* return Unit[] */ },
+  async load(locale) { /* return Map<id, StoredEntry> */ },
+  async write(locale, entries) { /* persist Map<id, StoredEntry> */ },
+};
+```
+
+Unit: `{ id: string, sourceText: string, kind: "markdown"|"text"|"html", context?: string }`
+- `id` must be stable across runs (a file path, a data key). It keys the stored translation.
+
+StoredEntry (written and read back by the adapter, hashes stored inline per the spec):
+`{ text: string, sourceHash: string, provenance: "machine"|"human", outputHash: string, stale?: boolean }`
+
+Rules the adapter owns:
+- `load` reads whatever inline representation the surface uses (frontmatter for `.md`,
+  `_sourceHash`/`_meta` for JSON) and reconstructs `StoredEntry`.
+- `write` persists it back in that same representation. `stale: true` entries keep their
+  existing `text` and should surface a review marker in the file (e.g. a frontmatter flag).
+- Adapters never call the model. They only extract and persist. `core.mjs` owns gating.