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/game_state.gd

@@ -66,6 +66,12 @@ var gauntlet_mode: bool = false  # true when snapshot includes gauntlet_mode fla
 # the server's "insert_active" snapshot field, disabling all z-layer-6 UI.
 var insert_active: bool = true
 
+# #175: World seed for deterministic simulation (D-010, D-029).
+# Set by SessionManager.new_game(), sent to server via StartupMessage in SimBridge.
+# Same seed → same EntanglementConfig → same NPC population across playthroughs.
+# Persists for the session lifetime; not overwritten by apply_snapshot().
+var world_seed: int = 0
+
 # #507: RNG seed for replay determinism — populated from snapshot "rng_seed" field.
 # Null in v0.1 (server does not yet send this field; protocol change required).
 var rng_seed: Variant = null

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
 
 

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

client/scripts/protocol/protocol.gd

@@ -427,6 +427,18 @@ static func _decode_enum_variant(raw) -> Dictionary:
 
 # -- Encode: GDScript types → bytes to server ----------------------------------
 
+## Encode a StartupMessage to MessagePack bytes (#175).
+## Sent by the client immediately after handshake validation.
+## Server reads this to initialize SimRng with the world seed (D-010, D-029).
+static func encode_startup_message(world_seed: int) -> PackedByteArray:
+	var msg := {"world_seed": world_seed}
+	var result = Messagepack.encode(msg)
+	if result.status != null:
+		push_error("Protocol: startup message encode failed: %s" % result.status)
+		return PackedByteArray()
+	return result.value
+
+
 ## Encode a PlayerInput to MessagePack bytes.
 ## action_name: one of "MoveNorth", "MoveSouth", "MoveEast", "MoveWest",
 ##              "Interact", "UsePerceptionMode", "Pause", "Unpause"