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/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);
+  });
+});