feat(client): add live server mode with make game target

Add SR_LIVE=1 environment variable to switch SimBridge from test mode
to TCP connection. Default behavior unchanged (test mode).

- sim_bridge.gd: read SR_LIVE env var instead of hardcoded test_mode
- game_state.gd: find player entity by kind.variant == "Player" instead
  of hardcoded entity_id 1 (real server assigns different IDs)
- Makefile: add 'make game' (builds server, starts it, launches client
  with SR_LIVE=1, kills server on exit) and 'make stop' helper

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/autoloads/sim_bridge.gd

@@ -4,7 +4,7 @@ extends Node
 enum ConnectionState { DISCONNECTED, CONNECTING, CONNECTED, ERROR }
 
 var state: ConnectionState = ConnectionState.DISCONNECTED
-var test_mode: bool = true  # Enable test mode for development without Rust server
+var test_mode: bool = OS.get_environment("SR_LIVE") != "1"  # SR_LIVE=1 connects to real server
 var _test_tick: int = 0
 var _test_player_pos: Vector2i = Vector2i(10, 10)
 var _test_facing: String = "North"