T6: Weapon switching and ammo through a service #19

Parent

Restructure spec: #13 (RESTRUCTURE_PLAN.md)

What to build

WeaponService owning per-brother weapon slots, switching, and ammo facts in WeaponsState; a single ammo listener replaces the per-switch signal reconnect dance in the input handler; the ammo API duplicated across weapons and the medkit is unified. Knockback, recoil, and shot visuals stay on the weapon node. Behavior is identical and ammo/switching rules are headlessly testable.

Acceptance criteria

• Next/prev weapon, ammo expenditure, empty-autoswitch, and ammo pickups behave exactly as they do today
• Ammo events flow through one subscription, not per-switch reconnect
• Ammo handling verified by tests; the duplicated ammo API is removed

Blocked by

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