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/ui/mini_map.gd

@@ -83,8 +83,8 @@ func _process(_delta: float) -> void:
 	if _rooms_data.is_empty():
 		return
 
-	# Find player if not cached
-	if _player == null:
+	# Find player if not cached or freed
+	if _player == null or not is_instance_valid(_player):
 		_player = get_tree().get_first_node_in_group("player")
 		if _player == null:
 			return