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/transport.ts

@@ -97,8 +97,8 @@ export function createTransport(baseUrl: string): GatewayClient {
     async gameHistory(id) {
       return codec.decodeHistory(await exec('game.history', codec.encodeGameAction(id)));
     },
-    async submitPlay(id, dir, tiles, variant) {
-      return codec.decodeMoveResult(await exec('game.submit_play', codec.encodeSubmitPlay(id, dir, tiles, variant)));
+    async submitPlay(id, tiles, variant) {
+      return codec.decodeMoveResult(await exec('game.submit_play', codec.encodeSubmitPlay(id, tiles, variant)));
     },
     async pass(id) {
       return codec.decodeMoveResult(await exec('game.pass', codec.encodeGameAction(id)));
@@ -112,8 +112,8 @@ export function createTransport(baseUrl: string): GatewayClient {
     async hint(id) {
       return codec.decodeHintResult(await exec('game.hint', codec.encodeGameAction(id)));
     },
-    async evaluate(id, dir, tiles, variant) {
-      return codec.decodeEvalResult(await exec('game.evaluate', codec.encodeEval(id, dir, tiles, variant)));
+    async evaluate(id, tiles, variant) {
+      return codec.decodeEvalResult(await exec('game.evaluate', codec.encodeEval(id, tiles, variant)));
     },
     async checkWord(id, word, variant) {
       return codec.decodeWordCheck(await exec('game.check_word', codec.encodeCheckWord(id, word, variant)));