s&box shared ammo pool: NetDictionary keyed by resource path, [Sync(FromHost)], optimistic client returns

Weapons in s&box that share an ammo type (e.g. all 9mm pistols) need a shared ammo pool on the player. Developers often store ammo per-weapon, which means picking up a second pistol doesn't add to the first's reserve. The correct pattern uses a shared pool keyed by ammo resource path.