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/placement.test.ts

@@ -2,7 +2,6 @@ import { describe, expect, it } from 'vitest';
 import {
   BLANK,
   cellOccupied,
-  direction,
   isBlankSlot,
   newPlacement,
   place,
@@ -47,23 +46,11 @@ describe('placement state machine', () => {
     expect(reset(place(p, 0, 7, 7)).pending).toHaveLength(0);
   });
 
-  it('infers direction H for a row, V for a column, null for a single tile', () => {
-    let h = place(newPlacement(rack), 0, 7, 7);
-    h = place(h, 1, 7, 8);
-    expect(direction(h)).toBe('H');
-    let v = place(newPlacement(rack), 0, 7, 7);
-    v = place(v, 1, 8, 7);
-    expect(direction(v)).toBe('V');
-    expect(direction(place(newPlacement(rack), 0, 7, 7))).toBeNull();
-  });
-
-  it('builds a sorted submit payload and honours a direction override', () => {
+  it('builds a sorted submit payload and returns null when empty', () => {
     let p = place(newPlacement(rack), 1, 7, 9);
     p = place(p, 0, 7, 7);
     const sub = toSubmit(p);
-    expect(sub?.dir).toBe('H');
     expect(sub?.tiles.map((t) => t.col)).toEqual([7, 9]);
-    expect(toSubmit(place(newPlacement(rack), 0, 7, 7), 'V')?.dir).toBe('V');
     expect(toSubmit(newPlacement(rack))).toBeNull();
   });
 
@@ -78,15 +65,8 @@ describe('placement state machine', () => {
     expect(isBlankSlot(newPlacement(rack), 0)).toBe(false);
   });
 
-  it('treats a non-linear placement as no inferred direction', () => {
-    let p = place(newPlacement(rack), 0, 7, 7);
-    p = place(p, 1, 8, 8); // diagonal
-    expect(direction(p)).toBeNull();
-  });
-
-  it('defaults a single-tile submit to H without an override', () => {
+  it('submits a single tile as a one-tile payload', () => {
     const sub = toSubmit(place(newPlacement(rack), 0, 7, 7));
-    expect(sub?.dir).toBe('H');
     expect(sub?.tiles).toHaveLength(1);
   });
 });