fix(dflash2): splice the draft cache across turns via a hidden gap

Live proof of the session store showed reuse working (348/377 and
500/532 prompt tokens, 0.33-0.36s delta prefill) but the draft cache
was dropped at every checkpoint: its self-trim runs at cycle start, so
at exit it always trails the target by the final cycle's kept tokens
and the alignment check refused it. A rebuilt draft then sees only the
delta as context, which cut acceptance from 65.4% to 29.7% on a
32-token delta.

The exit-time hidden always starts at the draft's offset on every exit
path, so the trailing positions are exactly hidden[:, :gap]. Store that
slice with the entry and prepend it to the delta hidden at resume time;
the drafter keeps its conversation history.