Backend infers play direction; UI previews words and gates submit on legality

A single tile that only extended a word perpendicular to the client-declared
direction was rejected: the UI always sent dir=H for one-tile plays (the
dirOverride/Controls toggle was orphaned in the Stage 7 game rework), so placing
"А" above "БАК" to form "АБАК" failed the solver's main-word-length check even
though the word is in the dictionary.

Make the backend infer a play's orientation from the placed tiles and the board
(internal/engine.resolveDirection): two or more tiles by the line they share, a
lone tile by the axis it abuts (longer word wins, horizontal on a tie). Direction
becomes an output, not an input: drop dir from the SubmitPlay/Eval wire requests
and add it to EvalResult. Journal replay keeps trusting the stored "H"/"V"
(SubmitPlayDir) so a rebuilt game matches the one committed.

UI: stop computing/sending direction; the preview now shows the words a move
forms with its total score (game.previewWords); the make-move control is disabled
until the play is confirmed legal; the "your turn" label hides while tiles are
pending. Delete the orphaned Controls.svelte.

Regenerate the FlatBuffers bindings (Go + TS) and update the gateway transcode
and the loadtest edge client to the new contract. Bake the decision into
ARCHITECTURE.md (§5/§9.1), FUNCTIONAL.md (+ _ru) and the backend README.

ui/src/lib/mock/client.ts

@@ -205,7 +205,7 @@ export class MockGateway implements GatewayClient {
     return { gameId, moves: structuredClone(g.moves) };
   }
 
-  async submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], _variant: Variant): Promise<MoveResult> {
+  async submitPlay(gameId: string, tiles: PlacedTile[], _variant: Variant): Promise<MoveResult> {
     const g = this.game(gameId);
     const seat = this.mySeat(g);
     if (g.view.toMove !== seat) throw new GatewayError('not_your_turn');
@@ -213,6 +213,7 @@ export class MockGateway implements GatewayClient {
     let score = tiles.reduce((s, t) => s + valueForLetter(variant, t.blank ? '?' : t.letter), 0);
     if (tiles.length === 7) score += 50;
     const total = g.view.seats[seat].score + score;
+    const dir = new Set(tiles.map((t) => t.row)).size === 1 ? 'H' : 'V';
     const move = {
       player: seat,
       action: 'play' as const,
@@ -311,12 +312,13 @@ export class MockGateway implements GatewayClient {
     };
   }
 
-  async evaluate(gameId: string, _dir: 'H' | 'V', tiles: PlacedTile[], _variant: Variant): Promise<EvalResult> {
+  async evaluate(gameId: string, tiles: PlacedTile[], _variant: Variant): Promise<EvalResult> {
     const g = this.game(gameId);
-    if (tiles.length === 0) return { legal: false, score: 0, words: [] };
+    if (tiles.length === 0) return { legal: false, score: 0, words: [], dir: '' };
     let score = tiles.reduce((s, t) => s + valueForLetter(g.view.variant, t.blank ? '?' : t.letter), 0);
     if (tiles.length === 7) score += 50;
-    return { legal: true, score, words: [tiles.map((t) => t.letter).join('')] };
+    const dir = new Set(tiles.map((t) => t.row)).size === 1 ? 'H' : 'V';
+    return { legal: true, score, words: [tiles.map((t) => t.letter).join('')], dir };
   }
 
   async checkWord(_gameId: string, word: string, _variant: Variant): Promise<WordCheckResult> {