Pulls the preserve-snap forward to step 2: it is the half with no UI and nothing
proposing today, and it needs nothing from the plate side but a fold that can
land last.
Makes the rule exact, because the direction is easy to get backwards and the
draft was vague about it. Slot k reads the latest preserved frame in
(d(k-1), d(k)], else d(k). The closure at native 13 in a 12-from-30 output is
recovered by slot 6, whose default is 15, reading 13 — NOT by slot 5 reaching
forward from 12, which would show it 17ms before the mouth shut and break the
invariant cadence_test already asserts.
Records where it goes, which is the part that was understated as "about thirty
lines". The slot interval exists only at clip.cljs:261 and group identity exists
only at symbol.cljs:399, so the interval has to be threaded down: two internal
signatures, not a drop-in. The rule's seat is the `(js/Math.floor lf)` default
`base-channel-frame` hands `pose/source-frame`, which leaves an explicit hand cut
beating a snap, as manual precedence requires.
And records the shortcut not to take: snapping at clip.cljs:261 needs no
threading and is wrong, because one native frame per output frame means the whole
picture reads 13 instead of 15 — a 67ms stale head to fix the mouth, fighting the
trace selection's own opinion about which head frame to show.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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