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/enemies/warden.gd

@@ -37,6 +37,9 @@ const MINE_AOE       := 70.0
 
 const FREEZE_DURATION := 1.5
 
+const SWEEP_PROJ_DAMAGE := 1.0
+const MINE_DAMAGE       := 1.0
+
 # ---------------------------------------------------------------------------
 # State
 # ---------------------------------------------------------------------------
@@ -215,7 +218,7 @@ func _spawn_sweep_proj(world_pos: Vector2, vel: Vector2) -> void:
 	area.add_child(cs)
 	area.body_entered.connect(func(body: Node) -> void:
 		if body.is_in_group("player"):
-			body.take_damage(1.0)
+			body.take_damage(SWEEP_PROJ_DAMAGE)
 			proj.queue_free())
 	proj.add_child(area)
 
@@ -321,7 +324,7 @@ func _explode_mine(entry: Dictionary) -> void:
 	if _player != null and is_instance_valid(_player):
 		var dist := node.global_position.distance_to(_player.global_position)
 		if dist <= MINE_AOE:
-			_player.take_damage(1.0)
+			_player.take_damage(MINE_DAMAGE)
 
 	# Brief visual flash for explosion (optional: could add particle later)
 	var flash := Polygon2D.new()