fix: address PR review — stale comments, mutex key normalization, test sync

- Update stale comments in event-buffer.ts (15s→3s) and socket.ts (5s→2s)
  to reflect actual timeout values
- Normalize mutex keys with jidNormalizedUser() for receiptMutex and
  notificationMutex to handle device suffix variants (e.g. user:5@s.whatsapp.net)
- Update offline-buffer-timeout.test.ts to match new 2s timeout value
  (was hardcoded at 5s, causing test/source divergence)
- Rejected suggestion to use attrs.id as fallback (false positive — attrs.id
  is a message ID, not a chat identifier; 'unknown' is the correct catch-all)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Utils/event-buffer.ts

@@ -54,7 +54,7 @@ export interface BufferConfig {
  */
 export function loadBufferConfig(): BufferConfig {
 	return {
-		// perf(inbound-latency): reduced from 15s → 5s so the safety auto-flush fires sooner.
+		// perf(inbound-latency): reduced from 15s → 3s so the safety auto-flush fires sooner.
 		// processNodeWithBuffer always calls ev.flush() explicitly (no-op for this timer),
 		// but socket.ts's offline-phase buffer and any stalled buffer benefit from the lower cap.
 		bufferTimeoutMs: parseInt(process.env.BAILEYS_BUFFER_TIMEOUT_MS || '3000', 10),