Make the mouth lead legible and assertable

"It feels like it does nothing" is indistinguishable from "it does nothing",
and at 24fps a lead of 1 is 42ms - small enough to reasonably doubt. So the
shift is now provable and visible rather than taken on trust:

- shiftIndex is a pure exported function with assertions covering identity,
  both directions, and clamping at each end
- the frame label always shows the mouth frame, not only when shifted, so the
  number can be watched diverging from f; non-zero leads also report in ms
- the slider readout carries an explicit + sign
- the stabilised pane draws the unshifted contour as a dark-green ghost when a
  lead is set, so the offset is something you can see

Also: leadIndex called opts() on every invocation - ~14 DOM reads, once per
strip thumbnail, so ~1000 per redraw at 74 frames. It reads a cached scalar now.

The "different pose" assertion checks the whole track rather than one pair:
synthetic poses hold for nine-frame beats, so a single pair can legitimately be
identical while the shift works correctly.

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

index.html

@@ -91,7 +91,7 @@
     <div class="panel">
       <h2>stabilised (head-local)</h2>
       <canvas id="cv-stab"></canvas>
-      <div class="legend">should sit still except the mouth · grey = held plate outline</div>
+      <div class="legend">should sit still except the mouth · grey = held plate outline<br>dark green ghost = unshifted mouth when lead ≠ 0</div>
     </div>
     <div class="panel">
       <h2>flat render — 320×200 indexed</h2>