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.test.ts

@@ -43,7 +43,6 @@ describe('codec', () => {
     // A placed blank carries its designated letter's index with the blank flag set.
     const buf = encodeSubmitPlay(
       'g1',
-      'H',
       [
         { row: 7, col: 7, letter: 'A', blank: false },
         { row: 7, col: 8, letter: 'B', blank: true },
@@ -52,7 +51,6 @@ describe('codec', () => {
     );
     const r = fb.SubmitPlayRequest.getRootAsSubmitPlayRequest(new ByteBuffer(buf));
     expect(r.gameId()).toBe('g1');
-    expect(r.dir()).toBe('H');
     expect(r.tilesLength()).toBe(2);
     expect(r.tiles(0)?.letter()).toBe(0);
     expect(r.tiles(1)?.letter()).toBe(1);