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

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

packages/shared/src/protocol.ts

@@ -1,5 +1,5 @@
 import type {
-  BoardView, Color, GameId, GameStatus, Mode, PlayerToken,
+  BoardView, CaptureTally, Color, GameId, GameStatus, Mode, PlayerToken,
   PromotionType, Square, EndReason,
 } from './types.js';
 import type { Announcement } from './moderator.js';
@@ -34,6 +34,7 @@ export type ServerMessage =
       mode: Mode;
       highlightingEnabled: boolean;
       opponentConnected: boolean;
+      captures: CaptureTally;
       aiOpponent?: { color: Color; brain: 'casual' | 'recon' };
     }
   | {
@@ -45,6 +46,7 @@ export type ServerMessage =
       drawOffer?: { from: Color } | null;
       endReason?: EndReason;
       winner?: Color | null;
+      captures: CaptureTally;
       aiOpponent?: { color: Color; brain: 'casual' | 'recon' };
     }
   | { type: 'peer-status'; color: Color; connected: boolean; graceUntil?: number }

packages/shared/src/types.ts

@@ -61,3 +61,12 @@ export function squareAt(fileIdx: number, rankIdx: number): Square | null {
   const r = String.fromCharCode('1'.charCodeAt(0) + rankIdx);
   return `${f}${r}` as Square;
 }
+
+/** Count of pieces by type — used for the capture tally. */
+export type PieceTally = Partial<Record<PieceType, number>>;
+
+/** Per-viewer capture tally: what you took, and what you lost. */
+export interface CaptureTally {
+  byYou: PieceTally;
+  byOpponent: PieceTally;
+}