### Purpose
Build a top-down zombie survival shooter game inspired by survivor.io that runs in the browser.

### Game Design

**Visual Style**
- Top-down view of arena/map
- Dark apocalyptic theme with muted colors
- Player character clearly visible in center
- Zombies approaching from all directions
- Blood splatter effects on zombie death
- Glowing XP gems scattered on ground

**Player Character**
- Circular or simple humanoid sprite
- Always faces direction of nearest enemy
- Auto-shoots projectiles at nearest zombie
- Health bar displayed above player
- Brief invincibility frames when hit (flashing)
- Dies when health reaches zero

**Weapons & Shooting**
- Auto-aim: Always targets nearest zombie
- Auto-fire: Shoots automatically at fire rate interval
- Starting weapon: Basic pistol (single shot)
- Projectiles travel toward target, can hit other zombies in path
- Visual muzzle flash on shoot

**Zombie Types**
- Basic zombie (green): Slow, low HP, common
- Fast zombie (red): Quick movement, low HP
- Tank zombie (purple): Slow, high HP, deals more damage
- Spawns increase each wave

**XP & Leveling System**
- Zombies drop XP gems on death (green glowing orbs)
- Walk over gems to collect
- XP bar fills at top of screen
- When XP bar full: LEVEL UP!
- Level up pauses game, shows upgrade selection

**Upgrade Selection (on Level Up)**
- Display 3 random upgrade options
- Player picks one to apply
- Upgrades stack and combine
- Options include:
  - Damage +20%
  - Fire rate +15%
  - Move speed +10%
  - Max HP +25
  - Projectile size +20%
  - Piercing shot (hits multiple enemies)
  - Spread shot (fires 3 projectiles)
  - Orbit shield (rotating projectiles around player)

**Wave System**
- Wave number displayed on screen
- Each wave: zombies spawn for 30-60 seconds
- Brief rest between waves (5 seconds)
- Each wave: more zombies, more variety, faster spawns
- Boss zombie every 5 waves (huge, high HP)

**Scoring & Game Over**
- Score = zombies killed + waves survived bonus
- High score saved to localStorage
- Game over when player HP reaches zero
- Show stats: zombies killed, waves survived, time alive

**Game Flow**
- Start screen with "SURVIVE" title
- Press start to begin Wave 1
- Survive as long as possible
- Game over screen with stats
- "PLAY AGAIN" option

### Controls

**Keyboard**
- WASD: Move in 8 directions
- Enter: Start game / Confirm selection
- 1/2/3 keys: Select upgrade option (during level up)

**Gamepad Support (IMPORTANT)**
The entire game must be playable with a gamepad controller, including menus:
- Use the Gamepad API: https://developer.mozilla.org/en-US/docs/Web/API/Gamepad_API
- Left analog stick: Move player
- D-pad: Navigate upgrade selection (left/right)
- A button (buttons[0]): Confirm upgrade selection
- A button on start screen: Begin game
- A button on game over: Restart
- Start button (buttons[9]): Pause game
- Show "Gamepad Connected" message when controller detected
- Poll gamepad state in game loop using `navigator.getGamepads()`

### Technical Notes
- Use HTML5 Canvas for rendering
- 60 FPS game loop with requestAnimationFrame
- Object pooling for projectiles and zombies (performance)
- Spawn zombies outside visible screen, move toward player
- Simple collision detection (circle-circle)
- Responsive canvas that scales to fit screen
- No external dependencies needed