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.ts

@@ -4,7 +4,7 @@
 // payload. It is board-agnostic (the gateway/engine does full legality validation at
 // submit), which keeps it trivially unit-testable.
 
-import type { Direction, Tile } from './model';
+import type { Tile } from './model';
 import type { PlacedTile } from './client';
 
 export interface PendingTile {
@@ -119,30 +119,13 @@ export function reorderIndices(n: number, from: number, toSlot: number): number[
   return order;
 }
 
-/**
- * direction infers the play orientation from the pending tiles: H if they share a row,
- * V if they share a column, null if a single tile (ambiguous) or non-linear (invalid).
- */
-export function direction(p: Placement): Direction | null {
-  if (p.pending.length < 2) return null;
-  const rows = new Set(p.pending.map((t) => t.row));
-  const cols = new Set(p.pending.map((t) => t.col));
-  if (rows.size === 1 && cols.size === p.pending.length) return 'H';
-  if (cols.size === 1 && rows.size === p.pending.length) return 'V';
-  return null;
-}
-
-/** toSubmit builds the submit payload. dirOverride resolves a single-tile play, where
- * the orientation cannot be inferred; otherwise the inferred direction is used. */
-export function toSubmit(
-  p: Placement,
-  dirOverride?: Direction,
-): { dir: Direction; tiles: PlacedTile[] } | null {
+/** toSubmit builds the submit payload: the placed tiles in board order. The backend
+ * infers the play's orientation from the tiles and the board, so none is sent. */
+export function toSubmit(p: Placement): { tiles: PlacedTile[] } | null {
   if (p.pending.length === 0) return null;
-  const dir = dirOverride ?? direction(p) ?? 'H';
   const tiles: PlacedTile[] = p.pending
     .slice()
     .sort((a, b) => a.row - b.row || a.col - b.col)
     .map((t) => ({ row: t.row, col: t.col, letter: t.letter, blank: t.blank }));
-  return { dir, tiles };
+  return { tiles };
 }