feat: Machine Gun Pickup power-up

[gpsanant]

Machine Gun Pickup

Adds a machine gun power-up pickup that spawns once per match at a random location inside the arena polygon.

Changes

• server/game.js: Pickup spawning in startRound(), collection detection via checkPickupCollection(), reduced cooldown via hasMachineGun flag in tryShoot(), pickup data in getState() broadcast, reset in resetForNextRound()
• client/client.js: Renders pickup as distinct orange icon with minigun barrel lines and MG label
• server/npc.js: NPCs initialized with hasMachineGun: false for pickup collection support
• test/game.test.js: 10 new comprehensive tests covering all pickup mechanics

Acceptance Criteria

1. Spawns at random valid position inside arena polygon once per match
2. Rendered on all clients as distinct visible icon
3. Collected when player moves within PLAYER_RADIUS proximity
4. Collecting player shoot cooldown reduced 50% (300ms to 150ms)
5. Game state broadcast includes pickup data (position, collected status)
6. Only one pickup spawns per match
7. NPCs can collect the pickup
8. Fire rate bonus resets at start of next round

All tests pass (0 failures).