The snap reads back into the slot's own gap, per pose group

Step 2 of docs/frame-selection.md, which f7e16e5 planned and left unbuilt: the
preserve-snap, and nothing of the plate side. `pose/snapped-frame` is the whole
rule — the latest mark in `(lo, hi]`, and `hi` when there is none — where `hi`
is the native frame the slot defaults to and `lo` is the one the slot before it
defaulted to. So the interval is exactly the frames this slot is the first to
cover, which are exactly the ones the grid shows to nobody: it recovers a
dropped frame out of its own gap, can never read a frame another slot already
showed, and cannot reach past `hi`.

Backward only. A closure at native 13 in a 12-from-30 output is recovered by the
slot whose default is 15, reading 13 — not by the slot at 12 reaching forward,
which would show the mouth shut 17ms before it did and break the no-lead
invariant `cadence_test` asserts over every grid and native pair. That test now
covers the snap too.

Seated as the DEFAULT pose that `pose/source-frame` reaches, so an explicit hand
cut beats a snap with nothing having to say so, and per pose group rather than
at the slot: snapping where the grid becomes native is one frame for the whole
picture, so a head would go two frames stale to fix one mouth. Groups exist only
in `symbol/base-channel-frame`, which is why the interval is threaded that far
down — `(:pre parent)` carried beside `(:f parent)` through the same time maps,
so an ancestor's exposure fold or retime is already in it.

The marks are the `[:vis]` cuts `flow/freeze` already stores, with their
thresholds and hysteresis already decided: no new signal, no new stored field.
A whitelist of `:roto/mouth-aperture` and `:roto/blink` and not a test for
`:generated`, because a skipped frame, a hidden feature and an absent
measurement are three different facts — snapping onto the frames a teeth contour
happened to be missing on is the cadence being dragged about by an absence.

Off is the default and needs no second code path: an opts map that says nothing
gets the behaviour it got before the snap existed. `:snap` is asked about the
SYMBOL, because the cuts are the face's own nodes' and every placement of one
face has the same ones.

The switch is `performance · <face>` in the inspector, and the readout says it
is the stage only. The document setting docs/frame-selection.md specifies wants
a leaf and a round trip of its own; until then this is `[:ui :smart]`, not
undoable, not synced, and unable to reach an export.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/time.md

@@ -23,8 +23,9 @@ sample-fps fields are gone. Existing exposure, trace choices and per-instance
 pose tracks remain available: a pose track can hold a chosen closed-mouth frame
 without deleting its neighboring measurements. Those choices stay in native
 frames when output fps changes. Automatic content-aware frame selection is not
-implemented. An event between output frames appears on the next output frame;
-it cannot create an extra frame in a 12fps output.
+implemented; [frame-selection.md](frame-selection.md) is how it should be. An
+event between output frames appears on the next output frame; it cannot create
+an extra frame in a 12fps output.
 
 Audio uses continuous time through the same derived placement maps, without
 picture floors or holds. Frame-rate units cancel before Web Audio playbackRate