fix: promotion dialog only fires for genuine pawn promotions

The "Promote pawn" dialog popped for any pawn "moved" toward the last
rank: the commit paths checked piece type + destination rank but never
the pawn's SOURCE rank. With the phantom layer now filling ranks 7-8
with tappable phantom pieces, tapping one (which falls through to the
real-move handler) while a real pawn was armed triggered the dialog for
a move no pawn could make — and for any phantom type, not just pawns.

Root cause: incomplete promotion detection, duplicated in Game.svelte
`onCommit` and the server's `isPromotionRequired`. Replaced with one
shared `isPromotionMove(piece, from, to)` — pawn, from the rank adjacent
to promotion, to the promotion rank, at most one file over — used by
both. 7 unit tests in packages/shared.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/client/src/lib/Game.svelte

@@ -9,7 +9,7 @@
   import { pieceGlyph } from './pieces.js';
   import { phantoms } from './stores/phantoms.svelte.js';
   import { phantomDrag } from './stores/phantom-drag.svelte.js';
-  import type { PromotionType, Square } from '@blind-chess/shared';
+  import { isPromotionMove, type PromotionType, type Square } from '@blind-chess/shared';
 
   interface Props { gameId: string; }
   let { gameId }: Props = $props();
@@ -40,13 +40,13 @@
   function onCommit(from: Square, to: Square) {
     const piece = game.state.view?.pieces[from];
     if (!piece) return;
-    // Promotion check (white pawn to rank 8, black pawn to rank 1).
-    if (piece.type === 'p') {
-      const rank = to[1];
-      if ((piece.color === 'w' && rank === '8') || (piece.color === 'b' && rank === '1')) {
-        pendingPromotion = { from, to };
-        return;
-      }
+    // A pawn promotes only from the rank adjacent to its promotion rank —
+    // isPromotionMove checks the source rank, destination rank, and file delta,
+    // so a pawn elsewhere "moved" toward the last rank no longer pops this
+    // dialog (which a click on a phantom-occupied back-rank square could do).
+    if (isPromotionMove(piece, from, to)) {
+      pendingPromotion = { from, to };
+      return;
     }
     game.commit(from, to);
   }

packages/server/src/commit.ts

@@ -1,6 +1,7 @@
 import type { Move } from 'chess.js';
 import {
   geometricMoves,
+  isPromotionMove,
   type Announcement,
   type Color,
   type Piece,
@@ -124,9 +125,6 @@ function chessJsLegalFrom(game: Game, from: Square): string[] {
 
 function isPromotionRequired(game: Game, from: Square, to: Square): boolean {
   const piece = game.chess.get(from);
-  if (!piece || piece.type !== 'p') return false;
-  const toRank = to[1];
-  if (piece.color === 'w' && toRank === '8') return true;
-  if (piece.color === 'b' && toRank === '1') return true;
-  return false;
+  if (!piece) return false;
+  return isPromotionMove({ color: piece.color, type: piece.type }, from, to);
 }

packages/shared/src/index.ts

@@ -3,3 +3,4 @@ export * from './moderator.js';
 export * from './protocol.js';
 export * from './geometric.js';
 export * from './phantoms.js';
+export * from './promotion.js';

packages/shared/src/promotion.ts

@@ -0,0 +1,23 @@
+import { fileIndex, type Piece, type Square } from './types.js';
+
+/**
+ * True iff moving `piece` from→to is a pawn promotion.
+ *
+ * A promotion is a pawn advancing from the rank adjacent to its promotion rank
+ * onto the promotion rank (7→8 for White, 2→1 for Black), at most one file
+ * over — a straight push or a diagonal capture.
+ *
+ * Checking only the piece type and the destination rank — as the move-commit
+ * paths previously did — wrongly flags e.g. a pawn on the 2nd rank "moved" to
+ * the 8th, popping the promotion dialog for a move no pawn could ever make.
+ */
+export function isPromotionMove(
+  piece: Piece | undefined,
+  from: Square,
+  to: Square,
+): boolean {
+  if (!piece || piece.type !== 'p') return false;
+  if (Math.abs(fileIndex(from) - fileIndex(to)) > 1) return false;
+  if (piece.color === 'w') return from[1] === '7' && to[1] === '8';
+  return from[1] === '2' && to[1] === '1';
+}

packages/shared/test/promotion.test.ts

@@ -0,0 +1,42 @@
+import { describe, it, expect } from 'vitest';
+import { isPromotionMove } from '../src/promotion.js';
+import type { Piece } from '../src/types.js';
+
+const wp: Piece = { color: 'w', type: 'p' };
+const bp: Piece = { color: 'b', type: 'p' };
+const wn: Piece = { color: 'w', type: 'n' };
+
+describe('isPromotionMove', () => {
+  it('white pawn from the 7th rank to the 8th is a promotion', () => {
+    expect(isPromotionMove(wp, 'e7', 'e8')).toBe(true);
+    expect(isPromotionMove(wp, 'd7', 'e8')).toBe(true); // capture-promotion
+  });
+
+  it('black pawn from the 2nd rank to the 1st is a promotion', () => {
+    expect(isPromotionMove(bp, 'e2', 'e1')).toBe(true);
+    expect(isPromotionMove(bp, 'd2', 'c1')).toBe(true);
+  });
+
+  it('a pawn NOT on the rank adjacent to promotion is not a promotion', () => {
+    expect(isPromotionMove(wp, 'e2', 'e8')).toBe(false); // the reported bug
+    expect(isPromotionMove(wp, 'e5', 'e8')).toBe(false);
+    expect(isPromotionMove(bp, 'e7', 'e1')).toBe(false);
+  });
+
+  it('an ordinary pawn move is not a promotion', () => {
+    expect(isPromotionMove(wp, 'e2', 'e4')).toBe(false);
+    expect(isPromotionMove(bp, 'e7', 'e5')).toBe(false);
+  });
+
+  it('a non-pawn reaching the last rank is not a promotion', () => {
+    expect(isPromotionMove(wn, 'g6', 'e8')).toBe(false);
+  });
+
+  it('a pawn move spanning more than one file is not a promotion', () => {
+    expect(isPromotionMove(wp, 'a7', 'h8')).toBe(false);
+  });
+
+  it('an undefined piece is not a promotion', () => {
+    expect(isPromotionMove(undefined, 'e7', 'e8')).toBe(false);
+  });
+});