feat(simulation): add world_seed IPC and EntanglementConfig (#175, #178)

Implements the StartupMessage protocol: client generates world_seed in
SessionManager.new_game(), sends it after handshake, server uses it to
seed SimRng and sample EntanglementConfig.

EntanglementConfig samples flat ∈ [25,35]%, intrigue ∈ [15,25]%, mundane
as remainder (D-029). Same seed produces identical config (D-010
determinism). Different seeds produce distinct configs in ≥90% of pairs.

Protocol flow: HandshakeMessage (server→client) → StartupMessage with
world_seed (client→server) → SimRng initialization → tick loop.

10 Rust tests (determinism, variation, bounds, sum invariant).
9 GDScript test stubs + 2 encode tests for client-side pipeline.

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

server/src/bridge/mod.rs

@@ -40,6 +40,11 @@ pub trait SimBridge: Send + Sync {
     /// any ObserverSnapshot is sent.
     fn send_handshake(&self) -> Result<(), BridgeError>;
 
+    /// Receive the client's startup message containing the world seed (#175).
+    /// Called exactly once, after send_handshake(), before entering the tick loop.
+    /// Blocks until the client sends the message.
+    fn receive_startup(&self) -> Result<StartupMessage, BridgeError>;
+
     /// Send an observer snapshot to the client
     fn send_snapshot(&self, snapshot: &ObserverSnapshot) -> Result<(), BridgeError>;
 
@@ -64,6 +69,10 @@ impl BridgeResource {
         self.inner.send_handshake()
     }
 
+    pub fn receive_startup(&self) -> Result<StartupMessage, BridgeError> {
+        self.inner.receive_startup()
+    }
+
     pub fn send_snapshot(&self, snapshot: &ObserverSnapshot) -> Result<(), BridgeError> {
         self.inner.send_snapshot(snapshot)
     }