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/tests/layer3.rs

@@ -84,7 +84,12 @@ fn server_subprocess_sends_snapshot_on_connect() {
         handshake.protocol_version, PROTOCOL_VERSION
     );
 
-    // 5. Send one PlayerInput (idle tick 0)
+    // 5. Send StartupMessage with world_seed (#175)
+    let startup = StartupMessage { world_seed: 42 };
+    let startup_payload = rmp_serde::to_vec_named(&startup).expect("serialize StartupMessage");
+    write_framed(&mut writer, &startup_payload).expect("send StartupMessage to server");
+
+    // 6. Send one PlayerInput (idle tick 0)
     let inputs = vec![PlayerInput {
         tick: 0,
         action: PlayerAction::MoveNorth,
@@ -92,14 +97,14 @@ fn server_subprocess_sends_snapshot_on_connect() {
     let payload = rmp_serde::to_vec_named(&inputs).expect("serialize PlayerInput");
     write_framed(&mut writer, &payload).expect("send PlayerInput to server");
 
-    // 6. Read one ObserverSnapshot
+    // 7. Read one ObserverSnapshot
     let response = read_framed(&mut reader)
         .expect("read snapshot frame")
         .expect("server closed connection before sending snapshot");
     let snapshot: ObserverSnapshot =
         rmp_serde::from_slice(&response).expect("deserialize ObserverSnapshot");
 
-    // 7. Assert protocol correctness (D-020)
+    // 8. Assert protocol correctness (D-020)
     assert_eq!(
         snapshot.version, PROTOCOL_VERSION,
         "protocol version mismatch: got {}, expected {}",
@@ -117,7 +122,7 @@ fn server_subprocess_sends_snapshot_on_connect() {
         .any(|e| matches!(e.kind, EntityKind::Player));
     assert!(has_player, "snapshot must contain a Player entity");
 
-    // 8. Clean up: drop connection so the server exits its game loop
+    // 9. Clean up: drop connection so the server exits its game loop
     drop(reader);
     drop(writer);