feat(server): per-viewer capture tally on joined and update messages

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

packages/server/test/unit/captures.test.ts

@@ -0,0 +1,32 @@
+import { describe, it, expect } from 'vitest';
+import { captureTally } from '../../src/captures.js';
+import type { Game, MoveRecord } from '../../src/state.js';
+import type { Color, PieceType } from '@blind-chess/shared';
+
+function rec(by: Color, capturedPieceType?: PieceType): MoveRecord {
+  return {
+    ply: 1, by, from: 'e2', to: 'e4', san: 'e4',
+    capturedPieceType, flags: {}, at: 0,
+  };
+}
+
+describe('captureTally', () => {
+  it('counts captures per viewer', () => {
+    const moveHistory: MoveRecord[] = [
+      rec('w', 'p'), rec('b'), rec('w', 'n'),
+      rec('b', 'p'), rec('w', 'p'),
+    ];
+    const game = { moveHistory } as unknown as Game;
+    expect(captureTally(game, 'w')).toEqual({
+      byYou: { p: 2, n: 1 }, byOpponent: { p: 1 },
+    });
+    expect(captureTally(game, 'b')).toEqual({
+      byYou: { p: 1 }, byOpponent: { p: 2, n: 1 },
+    });
+  });
+
+  it('returns empty tallies when there are no captures', () => {
+    const game = { moveHistory: [rec('w'), rec('b')] } as unknown as Game;
+    expect(captureTally(game, 'w')).toEqual({ byYou: {}, byOpponent: {} });
+  });
+});