feat: HUD overlay via UIScene — health bar, quick slots, inventory info (Phase 4.7)

UIScene renders as overlay on top of GameScene:
- Health bar (top-left) with color transitions green→yellow→red
- 4 quick slots (bottom-center) with active highlight, item symbols and counts
- Inventory weight/slots info (bottom-right)
- Controls hint (top-right)

GameScene pushes state to Phaser registry each frame.
Phase 4 (Player Systems) complete — 222 tests passing.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/config.ts

@@ -1,6 +1,7 @@
 import Phaser from 'phaser';
 import { BootScene } from './scenes/BootScene';
 import { GameScene } from './scenes/GameScene';
+import { UIScene } from './scenes/UIScene';
 
 export const GAME_WIDTH = 1280;
 export const GAME_HEIGHT = 720;
@@ -11,7 +12,7 @@ export const gameConfig: Phaser.Types.Core.GameConfig = {
   height: GAME_HEIGHT,
   backgroundColor: '#0a0a0a',
   parent: document.body,
-  scene: [BootScene, GameScene],
+  scene: [BootScene, GameScene, UIScene],
   physics: {
     default: 'arcade',
     arcade: {