T4: Wave and scoring through state and service

[Jon-Gear]

Parent

Restructure spec: #13 (RESTRUCTURE_PLAN.md)

What to build

The complete enemy-death-to-wave-advance path switched from global fields onto GameSessionState plus wave/score services: enemy death grants points, the scoreboard reads state, enemy counts and wave advancement run through the service instead of global counters. Same observable behavior, but this subsystem is now testable without a scene and stops writing global keys.

Acceptance criteria

• Killing enemies awards points, updates the board, decrements the enemy count, and advances waves exactly as they do today
• Wave and score logic lives in services tested against AppState fixtures
• No global key writes remain in this subsystem

Blocked by

• T3 AppSession composition root and AppState (T3: AppSession composition root and AppState #16)