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/codec.ts

@@ -87,7 +87,6 @@ export function encodeDraftSave(gameId: string, json: string): Uint8Array {
 
 export function encodeSubmitPlay(
   gameId: string,
-  dir: 'H' | 'V',
   tiles: PlacedTile[],
   variant: Variant,
 ): Uint8Array {
@@ -95,28 +94,19 @@ export function encodeSubmitPlay(
   const tileOffs = tiles.map((t) => buildPlayTile(b, t, variant));
   const vec = fb.SubmitPlayRequest.createTilesVector(b, tileOffs);
   const gid = b.createString(gameId);
-  const d = b.createString(dir);
   fb.SubmitPlayRequest.startSubmitPlayRequest(b);
   fb.SubmitPlayRequest.addGameId(b, gid);
-  fb.SubmitPlayRequest.addDir(b, d);
   fb.SubmitPlayRequest.addTiles(b, vec);
   return finish(b, fb.SubmitPlayRequest.endSubmitPlayRequest(b));
 }
 
-export function encodeEval(
-  gameId: string,
-  dir: 'H' | 'V',
-  tiles: PlacedTile[],
-  variant: Variant,
-): Uint8Array {
+export function encodeEval(gameId: string, tiles: PlacedTile[], variant: Variant): Uint8Array {
   const b = new Builder(256);
   const tileOffs = tiles.map((t) => buildPlayTile(b, t, variant));
   const vec = fb.EvalRequest.createTilesVector(b, tileOffs);
   const gid = b.createString(gameId);
-  const d = b.createString(dir);
   fb.EvalRequest.startEvalRequest(b);
   fb.EvalRequest.addGameId(b, gid);
-  fb.EvalRequest.addDir(b, d);
   fb.EvalRequest.addTiles(b, vec);
   return finish(b, fb.EvalRequest.endEvalRequest(b));
 }
@@ -377,7 +367,7 @@ export function decodeEvalResult(buf: Uint8Array): EvalResult {
   const r = fb.EvalResult.getRootAsEvalResult(new ByteBuffer(buf));
   const words: string[] = [];
   for (let i = 0; i < r.wordsLength(); i++) words.push(s(r.words(i)));
-  return { legal: r.legal(), score: r.score(), words };
+  return { legal: r.legal(), score: r.score(), words, dir: s(r.dir()) };
 }
 
 export function decodeWordCheck(buf: Uint8Array): WordCheckResult {