Length-prefixed MessagePack framing (4-byte BE length + payload),
LocalBridge struct implementing SimBridge trait over Unix domain
sockets, BridgeResource wrapper for ECS integration. Adds Io
error variant to BridgeError. Two integration tests verify
snapshot and input round-trips over real sockets.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
