fix(simulation): address bridge review feedback

Replace .lock().unwrap() with .expect("mutex poisoned") in LocalBridge
for clearer panic messages. Document 16MB MAX_MESSAGE_SIZE rationale
in framing.rs with entity count sizing analysis.

Addresses Hoshe PR review suggestions.

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

server/src/bridge/framing.rs

@@ -4,7 +4,10 @@
 
 use std::io::{self, Read, Write};
 
-/// Maximum message size: 16 MB
+/// Maximum message size: 16 MB.
+/// Sized for ObserverSnapshot with ~1000 entities (each ~40 bytes serialized),
+/// plus generous headroom for future field additions. A full-map dump of 10k
+/// entities would be ~400 KB, well within this limit.
 const MAX_MESSAGE_SIZE: u32 = 16 * 1024 * 1024;
 
 /// Write a length-prefixed message to a writer.