T5: Brothers, combat, and respawn through state and service #18

Parent

Restructure spec: #13 (RESTRUCTURE_PLAN.md)

What to build

BrothersState plus CombatService (damage, heal, and death reported through the service while entity health managers remain authoritative), RespawnService (eligibility and timing for reviving a downed brother), and an all-brothers-down event that reaches the flow seam instead of a global signal. The hurt, death, respawn-radius, and revive chain behaves exactly as today and is testable against state; split-screen and other systems read alive-facts from the brother state.

Acceptance criteria

• Hurt, death, respawn radius entry/exit, and revive behave exactly as they do today
• Combat and respawn logic verified by tests against BrothersState
• All-brothers-down reaches the flow seam, not a global signal

Blocked by

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