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/event_bus.gd

@@ -1,5 +1,18 @@
 extends Node
 
+# ---------------------------------------------------------------------------
+# Initialization Order Validation
+# ---------------------------------------------------------------------------
+# EventBus must be the first autoload. Other autoloads depend on it.
+# See project.godot for full dependency graph.
+static var _initialized: bool = false
+
+func _ready() -> void:
+	_initialized = true
+
+static func assert_ready() -> void:
+	assert(_initialized, "EventBus not initialized. Check autoload order in project.godot.")
+
 # ---------------------------------------------------------------------------
 # Save System
 # ---------------------------------------------------------------------------