feat: 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 `conformance` CI job (full-dictionary reader
parity plus a battery of plays across every variant and both cross-word rules,
including the inferred orientation). The server stays authoritative — submit_play
re-validates — so the local result is an advisory accelerator only.

- backend: Registry.DictBytes + an authed GET /api/v1/user/dict/{variant}/{version}
  (immutable) streaming the pinned per-game dawg; caddy routes /dict to the gateway.
- gateway: a session-gated /dict edge route proxying it; fetchDict on the transport.
- client: the dictionary loads on game open (low priority so it never starves the
  game on a slow link; aborted at a 5s cap or when leaving the game), is cached in
  IndexedDB (best-effort, self-healing on a rejected blob) and reused across
  sessions; a warm-up overlay covers a cold load, then the network preview is the
  fallback; a bad-connection breaker stops warming after repeated misses; the move
  preview cancels its in-flight request when the tiles change.
- parity generators backend/cmd/{dictgen,validategen} + gated Vitest suites, run in
  CI against the release dictionaries. A hidden debug readout lists the cached
  dictionaries + breaker state, and its reset clears the cache.
- docs: ARCHITECTURE §5, TESTING, UI_DESIGN, FUNCTIONAL (+ru).

docs/TESTING.md

@@ -21,6 +21,15 @@ tests or touching CI.
   mock for the friends screen (code issue/redeem, accept a request), the lobby
   invitations section, the stats screen, profile editing, and the GCG export's
   finished-only visibility.
+- **Local-eval conformance** — the client's on-device move preview (the ported
+  dawg reader + validator, `ui/src/lib/dict`) is checked byte-for-byte against the
+  authoritative Go engine. `backend/cmd/dictgen` and `backend/cmd/validategen` emit
+  golden vectors from the release dictionaries — every stored word plus a battery of
+  plays across both cross-word rules and all variants, each with the engine's
+  legality, score, words and inferred direction. The gated Vitest suites
+  (`ui/src/lib/dict/*.parity.test.ts`, skipped without their `DICT_*` env) replay
+  them and must agree exactly; the `conformance` CI job runs the whole loop, so a
+  drift in the port fails the merge.
 - **Engine** — correctness of scoring and move generation is owned
   by `scrabble-solver`'s own GCG-backed tests. `backend/internal/engine` adds, on
   top of the embedded solver: per-variant smoke tests (load all three committed