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).

backend/internal/server/handlers.go

@@ -90,6 +90,9 @@ func (s *Server) registerRoutes() {
 		u.GET("/games/:id/draft", s.handleGetDraft)
 		u.PUT("/games/:id/draft", s.handleSaveDraft)
 		u.POST("/games/:id/hide", s.handleHideGame)
+		// Raw dictionary download for the client-side local move preview, keyed by
+		// the game's pinned (variant, version); immutable, so cached hard.
+		u.GET("/dict/:variant/:version", s.handleDictBytes)
 	}
 	if s.feedback != nil {
 		u.POST("/feedback", s.handleFeedbackSubmit)

backend/internal/server/handlers_dict.go

@@ -0,0 +1,31 @@
+package server
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+
+	"scrabble/backend/internal/engine"
+)
+
+// handleDictBytes streams the raw serialized dictionary for a (variant, version)
+// pair so the client can validate and score moves locally — the local move
+// preview — instead of a network round trip per tile arrangement. It is reached
+// only through the gateway's session-gated /dict route (which resolves the
+// X-User-ID this group requires), and served with a long immutable cache lifetime
+// because a published dictionary version never changes. An unknown variant or a
+// version that is not resident is a 404.
+func (s *Server) handleDictBytes(c *gin.Context) {
+	variant, err := engine.ParseVariant(c.Param("variant"))
+	if err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "unknown variant"})
+		return
+	}
+	data, err := s.games.DictBytes(variant, c.Param("version"))
+	if err != nil {
+		c.JSON(http.StatusNotFound, gin.H{"error": "dictionary not found"})
+		return
+	}
+	c.Header("Cache-Control", "public, max-age=31536000, immutable")
+	c.Data(http.StatusOK, "application/octet-stream", data)
+}