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/main.rs

@@ -131,10 +131,19 @@ fn main() {
         std::process::exit(1);
     });
 
-    tracing::info!("Handshake sent, initializing simulation");
+    // Read client's startup message containing world_seed (#175).
+    // Client sends this immediately after validating the handshake.
+    let startup = bridge.receive_startup().unwrap_or_else(|e| {
+        tracing::error!("Failed to receive startup message: {}", e);
+        std::process::exit(1);
+    });
 
-    // RNG seed: test-mode defaults to 42 for deterministic replay
-    let seed = seed_flag.unwrap_or(if test_mode { 42 } else { 0 });
+    tracing::info!("Handshake complete, initializing simulation");
+
+    // RNG seed: --seed flag overrides client's world_seed (useful for testing).
+    // Production: client sends world_seed via StartupMessage (#175).
+    // Test mode default: 42 for deterministic replay.
+    let seed = seed_flag.unwrap_or(if test_mode { 42 } else { startup.world_seed });
 
     let mut app = App::new();
     app.add_plugins(SimulationPlugin);