New core module: multiplayer.js
Used by Chess, Ludo, Domino for all common multiplayer features:

1. OPPONENT BAR:
   - Avatar, name, rating, level
   - Green/yellow/red connection dot
   - Tap → popup menu (view profile, add friend, report)

2. SYNCED EMOTES:
   - sendEmote(matchId, type, key) → writes to game_state
   - checkForEmote(gameState, myId) → detects opponent's emote
   - onEmoteReceived(callback) → fires for UI to show floating emote

3. CONNECTION STATUS:
   - Green dot = connected (< 15s since last opponent action)
   - Yellow dot = weak (15-60s)
   - Red dot = disconnected (> 60s → can claim win)
   - startDisconnectWatch / stopDisconnectWatch

4. REMATCH:
   - requestRematch(matchId, type) → writes to game_state
   - checkForRematch(gameState, myId) → detects request

5. FRIEND ADD:
   - addFriendFromGame(opponentId) → sends friend request
   - Accessible from opponent bar tap menu

6. REPORT:
   - reportOpponent(opponentId, reason)

All games can import { renderOpponentBar, sendEmote, ... } from 'core/multiplayer.js'
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>