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.

tests/unit/scripts/i18n/fake-adapter.ts

@@ -0,0 +1,18 @@
+// tests/unit/scripts/i18n/fake-adapter.ts
+// In-memory adapter matching the contract in scripts/i18n/adapter-contract.md.
+export function makeFakeAdapter(units: Array<{ id: string; sourceText: string; kind?: string }>) {
+  const store = new Map<string, Map<string, any>>(); // locale -> (id -> entry)
+  return {
+    name: "fake",
+    async extract() {
+      return units.map((u) => ({ kind: "markdown", ...u }));
+    },
+    async load(locale: string) {
+      return new Map(store.get(locale) ?? new Map());
+    },
+    async write(locale: string, entries: Map<string, any>) {
+      store.set(locale, new Map(entries));
+    },
+    _store: store,
+  };
+}