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. */

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);

ui/src/lib/codec.ts

@@ -87,7 +87,6 @@ export function encodeDraftSave(gameId: string, json: string): Uint8Array {
 
 export function encodeSubmitPlay(
   gameId: string,
-  dir: 'H' | 'V',
   tiles: PlacedTile[],
   variant: Variant,
 ): Uint8Array {
@@ -95,28 +94,19 @@ export function encodeSubmitPlay(
   const tileOffs = tiles.map((t) => buildPlayTile(b, t, variant));
   const vec = fb.SubmitPlayRequest.createTilesVector(b, tileOffs);
   const gid = b.createString(gameId);
-  const d = b.createString(dir);
   fb.SubmitPlayRequest.startSubmitPlayRequest(b);
   fb.SubmitPlayRequest.addGameId(b, gid);
-  fb.SubmitPlayRequest.addDir(b, d);
   fb.SubmitPlayRequest.addTiles(b, vec);
   return finish(b, fb.SubmitPlayRequest.endSubmitPlayRequest(b));
 }
 
-export function encodeEval(
-  gameId: string,
-  dir: 'H' | 'V',
-  tiles: PlacedTile[],
-  variant: Variant,
-): Uint8Array {
+export function encodeEval(gameId: string, tiles: PlacedTile[], variant: Variant): Uint8Array {
   const b = new Builder(256);
   const tileOffs = tiles.map((t) => buildPlayTile(b, t, variant));
   const vec = fb.EvalRequest.createTilesVector(b, tileOffs);
   const gid = b.createString(gameId);
-  const d = b.createString(dir);
   fb.EvalRequest.startEvalRequest(b);
   fb.EvalRequest.addGameId(b, gid);
-  fb.EvalRequest.addDir(b, d);
   fb.EvalRequest.addTiles(b, vec);
   return finish(b, fb.EvalRequest.endEvalRequest(b));
 }
@@ -377,7 +367,7 @@ export function decodeEvalResult(buf: Uint8Array): EvalResult {
   const r = fb.EvalResult.getRootAsEvalResult(new ByteBuffer(buf));
   const words: string[] = [];
   for (let i = 0; i < r.wordsLength(); i++) words.push(s(r.words(i)));
-  return { legal: r.legal(), score: r.score(), words };
+  return { legal: r.legal(), score: r.score(), words, dir: s(r.dir()) };
 }
 
 export function decodeWordCheck(buf: Uint8Array): WordCheckResult {

ui/src/lib/i18n/en.ts

@@ -67,7 +67,7 @@ export const en = {
   'game.checkWord': 'Check word',
   'game.dictionary': 'Dictionary',
   'game.dropGame': 'Drop game',
-  'game.preview': 'Scores {n}',
+  'game.previewWords': '{words}: {n}',
   'game.previewIllegal': 'Not a legal move',
   'game.chooseBlank': 'Choose a letter for the blank',
   'game.exchangeTitle': 'Select tiles to exchange',
@@ -86,7 +86,6 @@ export const en = {
   'game.check': 'Check',
   'game.checkWait': 'Please wait a moment.',
   'game.noHintOptions': 'No options with your letters.',
-  'game.scores': 'Scores: {n}',
   'game.thinking': 'thinking…',
 
   'move.pass': 'pass',

ui/src/lib/i18n/ru.ts

@@ -68,7 +68,7 @@ export const ru: Record<MessageKey, string> = {
   'game.checkWord': 'Проверить слово',
   'game.dictionary': 'Словарь',
   'game.dropGame': 'Покинуть игру',
-  'game.preview': 'Очков: {n}',
+  'game.previewWords': '{words}: {n}',
   'game.previewIllegal': 'Недопустимый ход',
   'game.chooseBlank': 'Выберите букву для бланка',
   'game.exchangeTitle': 'Выберите фишки для обмена',
@@ -87,7 +87,6 @@ export const ru: Record<MessageKey, string> = {
   'game.check': 'Проверить',
   'game.checkWait': 'Секунду, пожалуйста.',
   'game.noHintOptions': 'Нет вариантов с вашим набором.',
-  'game.scores': 'Очков: {n}',
   'game.thinking': 'думает…',
 
   'move.pass': 'пас',

ui/src/lib/mock/client.ts

@@ -205,7 +205,7 @@ export class MockGateway implements GatewayClient {
     return { gameId, moves: structuredClone(g.moves) };
   }
 
-  async submitPlay(gameId: string, dir: 'H' | 'V', tiles: PlacedTile[], _variant: Variant): Promise<MoveResult> {
+  async submitPlay(gameId: string, tiles: PlacedTile[], _variant: Variant): Promise<MoveResult> {
     const g = this.game(gameId);
     const seat = this.mySeat(g);
     if (g.view.toMove !== seat) throw new GatewayError('not_your_turn');
@@ -213,6 +213,7 @@ export class MockGateway implements GatewayClient {
     let score = tiles.reduce((s, t) => s + valueForLetter(variant, t.blank ? '?' : t.letter), 0);
     if (tiles.length === 7) score += 50;
     const total = g.view.seats[seat].score + score;
+    const dir = new Set(tiles.map((t) => t.row)).size === 1 ? 'H' : 'V';
     const move = {
       player: seat,
       action: 'play' as const,
@@ -311,12 +312,13 @@ export class MockGateway implements GatewayClient {
     };
   }
 
-  async evaluate(gameId: string, _dir: 'H' | 'V', tiles: PlacedTile[], _variant: Variant): Promise<EvalResult> {
+  async evaluate(gameId: string, tiles: PlacedTile[], _variant: Variant): Promise<EvalResult> {
     const g = this.game(gameId);
-    if (tiles.length === 0) return { legal: false, score: 0, words: [] };
+    if (tiles.length === 0) return { legal: false, score: 0, words: [], dir: '' };
     let score = tiles.reduce((s, t) => s + valueForLetter(g.view.variant, t.blank ? '?' : t.letter), 0);
     if (tiles.length === 7) score += 50;
-    return { legal: true, score, words: [tiles.map((t) => t.letter).join('')] };
+    const dir = new Set(tiles.map((t) => t.row)).size === 1 ? 'H' : 'V';
+    return { legal: true, score, words: [tiles.map((t) => t.letter).join('')], dir };
   }
 
   async checkWord(_gameId: string, word: string, _variant: Variant): Promise<WordCheckResult> {

ui/src/lib/model.ts

@@ -84,6 +84,8 @@ export interface EvalResult {
   legal: boolean;
   score: number;
   words: string[];
+  /** Orientation the backend inferred for the play ("H"/"V"), empty when illegal. */
+  dir: string;
 }
 
 export interface WordCheckResult {

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);
   });
 });

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 };
 }

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)));