The vision prefill phase: one sequence's prompt from position start,
the prepared embeddings at 0 and its token ids after a reused prefix, at
the request's positions. phase and width are the scheduler's: the
planner's name for the piece (one position after a reused prefix appends
as a decode, more as a tail) and the class of its rows. Returns the hidden
of the prompt's last position.
Prepared native input with an indivisible prefill. The preparation owner retains its tensors; the graph's binding returns an owned final hidden row. Decode, sampling and row retirement use the ordinary execution group.