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/run_manager.gd

@@ -1,5 +1,8 @@
 extends Node
 
+func _ready() -> void:
+	EventBus.assert_ready()  # Verify autoload order
+
 var current_floor: int = 0
 var current_room_id: String = ""
 var run_active: bool = false