feat(ui): on-device move preview (local eval) with network fallback

Score and validate a tentative move on-device instead of a per-arrangement
network round trip. The dawg reader and the validate/score/direction slice of the
scrabble-solver engine are ported to TypeScript (ui/src/lib/dict), pinned
byte-for-byte to the Go engine by a new `conformance` CI job. The server stays
authoritative — submit_play re-validates — so the local result is an advisory
accelerator only; any cache miss, storage eviction or a bad-connection breaker
falls back to the network evaluate.

- backend: Registry.DictBytes + an authed GET /api/v1/user/dict/{variant}/{version}
  (immutable) streaming the pinned per-game dawg.
- gateway: a session-gated /dict edge route proxying it; fetchDict on the transport.
- client: an IndexedDB blob cache (best-effort, storage.persist()) + a loader
  (memory -> IndexedDB -> network, session-scoped bad-connection breaker) + a lobby
  prefetch (your-turn first); an adapter over the existing premiums/alphabet; a
  DictWarmup overlay while a cold dictionary loads (120ms flash-guard, 5s cap ->
  network); a ?nolocal flag; the DebugPanel reset also clears the dict cache.
- parity: generators backend/cmd/{dictgen,validategen} + gated Vitest suites, run
  in CI against the release dictionaries.
- docs: ARCHITECTURE §5, TESTING, UI_DESIGN, FUNCTIONAL (+ru).

docs/FUNCTIONAL.md

@@ -163,7 +163,10 @@ tile that extends
 an existing word (down a column or across a row) is accepted. A play is validated
 against the game's dictionary at submit time and scored; an unlimited preview
 reports the word(s) a tentative move would form and its score, or that it is not
-legal, and the move is offered for submission only once it is confirmed legal. The dictionary check tool is
+legal, and the move is offered for submission only once it is confirmed legal. The preview is
+computed **on-device** for an instant response once the game's dictionary has loaded — a
+brief warm-up shows on the first open otherwise — and falls back to the server whenever the
+local dictionary is unavailable. The dictionary check tool is
 unlimited and offers a complaint on any result; for a word it finds, it also links out to an
 external reference dictionary (gramota.ru for Russian games, scrabblewordfinder.org for
 English) to look it up. Hints are governed per game —