feat(simulation): add LocalBridge IPC over Unix socket (#78)

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>

server/src/bridge/mod.rs

@@ -3,7 +3,10 @@
 // MessagePack serialization for Rust<->Godot communication
 
 use bevy_app::prelude::*;
+use bevy_ecs::prelude::*;
 
+pub mod framing;
+pub mod local;
 pub mod types;
 pub use types::*;
 
@@ -14,6 +17,8 @@ pub enum BridgeError {
     Serialization(#[from] rmp_serde::encode::Error),
     #[error("deserialization error: {0}")]
     Deserialization(#[from] rmp_serde::decode::Error),
+    #[error("io error: {0}")]
+    Io(#[from] std::io::Error),
     #[error("transport error: {0}")]
     Transport(String),
 }
@@ -28,6 +33,28 @@ pub trait SimBridge: Send + Sync {
     fn receive_inputs(&self) -> Result<Vec<PlayerInput>, BridgeError>;
 }
 
+/// BridgeResource: Bevy Resource wrapper for SimBridge trait object
+#[derive(Resource)]
+pub struct BridgeResource {
+    inner: Box<dyn SimBridge>,
+}
+
+impl BridgeResource {
+    pub fn new(bridge: impl SimBridge + 'static) -> Self {
+        Self {
+            inner: Box::new(bridge),
+        }
+    }
+
+    pub fn send_snapshot(&self, snapshot: &ObserverSnapshot) -> Result<(), BridgeError> {
+        self.inner.send_snapshot(snapshot)
+    }
+
+    pub fn receive_inputs(&self) -> Result<Vec<PlayerInput>, BridgeError> {
+        self.inner.receive_inputs()
+    }
+}
+
 /// Bridge plugin for client-server communication
 /// Abstracts transport layer (LocalBridge/NetworkBridge)
 pub struct BridgePlugin;