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/tcp.rs

@@ -129,6 +129,28 @@ impl TcpBridge {
 }
 
 impl SimBridge for TcpBridge {
+    fn receive_startup(&self) -> Result<super::StartupMessage, BridgeError> {
+        let mut reader = self
+            .reader
+            .lock()
+            .map_err(|e| BridgeError::MutexPoisoned(format!("reader: {}", e)))?;
+        // Toggle to blocking for reliable startup message read.
+        // The client sends StartupMessage immediately after handshake validation,
+        // so this read should complete quickly.
+        reader.get_mut().set_nonblocking(false).map_err(BridgeError::Io)?;
+        let result = read_framed(reader.get_mut());
+        // Restore non-blocking for the tick loop
+        reader.get_mut().set_nonblocking(true).map_err(BridgeError::Io)?;
+        match result? {
+            Some(payload) => {
+                let msg: super::StartupMessage = rmp_serde::from_slice(&payload)?;
+                tracing::info!("received startup message: world_seed={}", msg.world_seed);
+                Ok(msg)
+            }
+            None => Err(BridgeError::Disconnected),
+        }
+    }
+
     fn send_handshake(&self) -> Result<(), BridgeError> {
         use super::types::{HandshakeMessage, PROTOCOL_VERSION};
         let msg = HandshakeMessage {