[TASK-11] UI System, HUD & Overlay Nodes #101

Description: Implement UISystem (the sole EventBus-to-HUD bridge), HUDNode, PowerBarNode, SkillSelectionViewController, and TurnHandoffOverlay. The HUD must be a child of SKCameraNode to remain fixed on screen.

Task Acceptance Criteria:

• HUDNode is attached to SKCameraNode and stays fixed during all camera movements.
• PowerBarNode.updateFill(_:) scales fill and transitions green→yellow→red per amplitude value.
• SkillSelectionViewController shows 3 skills (grayed if consumed), current effect values, and posts the correct EventBus event on any selection.
• TurnHandoffOverlay fades in on showTurnHandoff, shows correct player number, fades out on handoffDismissed.
• UISystem contains zero game logic — only event subscriptions and HUD update calls.

Prerequisites: TASK-01 (EventBus, GameTypes), TASK-03 (entity foundation), TASK-07 (DamageCycleManager for skill UI values).