Function with_current 

pub fn with_current<T: 'static, R>(f: impl FnOnce(&mut T) -> R) -> Option<R>

Run f with a mutable borrow of the currently-set host context. Returns None if LuaHost::eval isn’t on the stack.

Used inside the dispatch fn passed to LuaHost::new — call once per redis.call, do the kevy dispatch, return RESP bytes.