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/session_manager.gd

@@ -31,6 +31,12 @@ func new_game() -> String:
 			save_path, error_string(err)])
 		return ""
 	GameState.current_game_id = game_id
+
+	# #175: Generate world_seed for deterministic simulation (D-010, D-029).
+	# Uses randi() (u32) for a seed that maps cleanly to Rust u64 via MessagePack.
+	# 4 billion seeds is sufficient entropy for EntanglementConfig variation (D-029).
+	GameState.world_seed = rng.randi()
+
 	return game_id