readPublishedFrame

Borrows the latest copied render cache, or returns null before first publication.

Safe from any thread, including after closure; this does not request a new frame. The callback may read/copy borrowed primitive planes but must not mutate or retain the cache or its arrays. It must not close the session or reenter its mutation or publication APIs. Failures and non-local returns release the lease before escaping. A null result can also be the callback's own result. No lease or callback object is allocated for an inlined call; the publisher remains owned exclusively by session.