Function heal_entity

pub fn heal_entity(
    sink: &mut dyn FightSink,
    by_entity_id: Option<EntityId>,
    entity: &Entity,
    amount: f64,
    source: CombatSource,
) -> Result<Option<i64>, Error>

Native: heal entity, capping at its missing HP. Returns the applied heal (None if no heal was applied). by_entity_id and source are the breakdown attribution — who healed, and with what.