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/client.ts

@@ -74,12 +74,12 @@ export interface GatewayClient {
   // table), and gameState's includeAlphabet asks the server to embed that table.
   gameState(gameId: string, includeAlphabet: boolean): Promise<StateView>;
   gameHistory(gameId: string): Promise<History>;
-  submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], variant: Variant): Promise<MoveResult>;
+  submitPlay(gameId: string, tiles: PlacedTile[], variant: Variant): Promise<MoveResult>;
   pass(gameId: string): Promise<MoveResult>;
   exchange(gameId: string, tiles: string[], variant: Variant): Promise<MoveResult>;
   resign(gameId: string): Promise<MoveResult>;
   hint(gameId: string): Promise<HintResult>;
-  evaluate(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], variant: Variant): Promise<EvalResult>;
+  evaluate(gameId: string, tiles: PlacedTile[], variant: Variant): Promise<EvalResult>;
   checkWord(gameId: string, word: string, variant: Variant): Promise<WordCheckResult>;
   complaint(gameId: string, word: string, note: string): Promise<void>;
   /** Hide a finished game from the caller's own lobby list; per-account, irreversible. */