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/repeater.gd

@@ -65,7 +65,7 @@ func _physics_process(delta: float) -> void:
 		return
 	velocity = Vector2.ZERO
 	_do_fire(delta)
-	velocity *= _slow_factor
+	velocity *= get_slow_factor()
 	move_and_slide()
 
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