Fix critical bugs from code review

- room.gd: Add signal tracking and explicit cleanup in _exit_tree
- room.gd: Guard _on_enemy_died against race condition after room freed
- player.gd: Set _shield_ready = false on shield activation
- player.gd: Fix coolant zone lambda syntax error
- player.gd: Add apply_knockback() method for Hivemind pulse
- player.gd: Fix coolant zone memory leak (use Timer child)
- hud.gd: Remove deprecated boss bar code

scripts/game_manager.gd

@@ -16,6 +16,7 @@ const RunEndScreenScene = preload("res://scripts/ui/run_end_screen.gd")
 var _run_end_screen: CanvasLayer = null
 
 func _ready() -> void:
+	EventBus.assert_ready()  # Verify autoload order
 	EventBus.run_started.connect(_on_run_started)
 	EventBus.run_ended.connect(_on_run_ended)
 	call_deferred("_load_game")