Add browser-based Doom clone with raycasting engine

Built a complete FPS game playable in the browser using vanilla JS and
HTML5 Canvas. Features a raycasting renderer with 8 textured wall types,
3 weapons (pistol, shotgun, plasma gun), 3 enemy types with AI
(imp, demon, baron), 3 levels of increasing difficulty, HUD with
minimap, pickups, doors, explosive barrels, and procedural audio.
Includes GitHub Actions workflow for GitHub Pages deployment.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/constants.js

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+// Game constants
+const SCREEN_WIDTH = 640;
+const SCREEN_HEIGHT = 480;
+const TILE_SIZE = 64;
+const FOV = Math.PI / 3; // 60 degrees
+const HALF_FOV = FOV / 2;
+const NUM_RAYS = SCREEN_WIDTH;
+const DELTA_ANGLE = FOV / NUM_RAYS;
+const MAX_DEPTH = 20;
+const WALL_HEIGHT_SCALE = TILE_SIZE * 3;
+const PLAYER_SPEED = 3.0;
+const PLAYER_ROT_SPEED = 0.003;
+const PLAYER_SIZE = 0.3;
+const MOUSE_SENSITIVITY = 0.003;
+
+// Wall types
+const WALL_EMPTY = 0;
+const WALL_STONE = 1;
+const WALL_BRICK = 2;
+const WALL_METAL = 3;
+const WALL_TECH = 4;
+const WALL_DOOR = 5;
+const WALL_EXIT = 6;
+const WALL_WOOD = 7;
+const WALL_HELL = 8;
+
+// Entity types
+const ENTITY_ENEMY_IMP = 'imp';
+const ENTITY_ENEMY_DEMON = 'demon';
+const ENTITY_ENEMY_BARON = 'baron';
+const ENTITY_PICKUP_HEALTH = 'health';
+const ENTITY_PICKUP_ARMOR = 'armor';
+const ENTITY_PICKUP_AMMO = 'ammo';
+const ENTITY_PICKUP_SHOTGUN = 'shotgun_pickup';
+const ENTITY_PICKUP_PLASMA = 'plasma_pickup';
+const ENTITY_BARREL = 'barrel';
+const ENTITY_LAMP = 'lamp';
+
+// Game states
+const STATE_MENU = 'menu';
+const STATE_PLAYING = 'playing';
+const STATE_PAUSED = 'paused';
+const STATE_LEVEL_COMPLETE = 'level_complete';
+const STATE_GAME_OVER = 'game_over';
+const STATE_GAME_WON = 'game_won';