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/items/shop_item_pedestal.gd

@@ -73,11 +73,10 @@ func _attempt_purchase() -> void:
 	if player == null:
 		return
 
-	RunManager.collect_item(item)
+	RunManager.collect_item(item)  # Triggers item_collected signal -> player_stats recalculates
 	if item.heal_on_pickup > 0.0:
 		player.stats.heal(item.heal_on_pickup)
-	player.stats.recalculate()
-	EventBus.player_health_changed.emit(player.stats.current_health, player.stats.max_health)
+		EventBus.player_health_changed.emit(player.stats.current_health, player.stats.max_health)
 
 	_sold            = true
 	_player_in_range = false