feat: player entity with WASD movement, tile collision, camera follow (Phase 4.1)

Player spawns at walkable tile near map center. WASD controls movement
(150px/s, normalized diagonal). Tile collision with wall-sliding prevents
walking through acid pools, crystals, geysers. Camera follows player with
smooth lerp. 39 new tests (134 total).

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

src/scenes/BootScene.ts

@@ -30,7 +30,7 @@ export class BootScene extends Phaser.Scene {
 
     // Version
     this.add
-      .text(cx, cy + 80, 'v0.2.0 — Phase 2: ECS Foundation', {
+      .text(cx, cy + 80, 'v0.4.0 — Phase 4: Player Systems', {
         fontSize: '12px',
         color: '#333333',
         fontFamily: 'monospace',