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>

client/scripts/autoloads/sim_bridge.gd

@@ -231,6 +231,26 @@ func _process(delta: float) -> void:
 			_set_state(ConnectionState.ERROR)
 			return
 
+		# Send startup message with world_seed (#175, D-010/D-029).
+		# Server blocks waiting for this before entering the tick loop.
+		var startup_bytes := Protocol.encode_startup_message(GameState.world_seed)
+		if startup_bytes.size() > 0:
+			var send_err := _bridge.send_message(startup_bytes)
+			if send_err != OK:
+				var reason := "Failed to send startup message: %s" % error_string(send_err)
+				push_error("SimBridge: %s" % reason)
+				handshake_failed.emit(reason)
+				_bridge.disconnect_from_server()
+				_set_state(ConnectionState.ERROR)
+				return
+		else:
+			var reason := "Failed to encode startup message"
+			push_error("SimBridge: %s" % reason)
+			handshake_failed.emit(reason)
+			_bridge.disconnect_from_server()
+			_set_state(ConnectionState.ERROR)
+			return
+
 		handshake_complete.emit(server_version)
 		_set_state(ConnectionState.CONNECTED)
 		return