perf(messages): enable parallel message processing with KeyedMutex + robust fallback handling

This commit implements KeyedMutex for parallel message processing across different chats
while addressing Copilot PR #74 review concerns about edge case handling.

Key Changes:
1. Replace global messageMutex with KeyedMutex (per-chat locking)
   - src/Socket/chats.ts: Import makeKeyedMutex, update initialization
   - Messages from different chats now process in parallel
   - Messages within same chat maintain sequential order (preserves integrity)

2. Implement robust fallback chain for mutex keys
   - Primary: msg.key.remoteJid (always present in practice)
   - Secondary: msg.key.id (unique per message, enables parallelism in edge cases)
   - Tertiary: 'unknown' (serializes only truly malformed messages)

3. Add defensive logging for edge cases
   - Logs warnings when remoteJid is missing (should never happen)
   - Includes msg.key.id in logs for traceability

Performance Impact:
- Before: ALL messages serialized (10 messages from 10 chats = 10x latency)
- After: Parallel processing per chat (10 messages from 10 chats = ~1x latency)
- Edge case: Even if remoteJid missing, uses msg.key.id instead of serializing all

Safety Guarantees:
- ✅ Message order preserved within same conversation (interactive messages safe)
- ✅ No impact on buttons, lists, carousels, or any interactive message types
- ✅ Defensive programming prevents performance degradation in edge cases
- ✅ Logging enables debugging of unexpected scenarios

Addresses:
- Copilot PR #74 comments 1, 2, 4, 5 (fallback handling)
- Original performance issue (message delivery latency)

https://claude.ai/code/session_0149ZKk2ygmKCJTGu39Mr8oH

src/Socket/messages-recv.ts

@@ -1266,7 +1266,14 @@ export const makeMessagesRecvSocket = (config: SocketConfig) => {
 		}
 
 		try {
-			await messageMutex.mutex(async () => {
+			// Use KeyedMutex with remoteJid to allow parallel processing of messages from different chats
+			// while maintaining order for messages within the same chat
+			// Fallback: msg.key.id (unique per message) > 'unknown' (serializes all unknown messages)
+			const mutexKey = msg.key.remoteJid || (() => {
+				logger.warn({ msgId: msg.key.id, fromMe: msg.key.fromMe }, 'Missing remoteJid in message key, using msg.key.id as fallback')
+				return msg.key.id || 'unknown'
+			})()
+			await messageMutex.mutex(mutexKey, async () => {
 				await decrypt()
 				// message failed to decrypt
 				if (msg.messageStubType === proto.WebMessageInfo.StubType.CIPHERTEXT && msg.category !== 'peer') {