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>

js/pipeline.js

@@ -177,3 +177,12 @@ export function heldFrame(kept, f) {
   for (const k of kept) { if (k <= f) hit = k; else break; }
   return hit;
 }
+
+// Shift a performance track against the clock, clamped at the ends.
+//
+// Pure and exported so the shift can actually be asserted: "the slider feels
+// like it does nothing" is otherwise indistinguishable from "the slider does
+// nothing", and at 24fps a lead of 1 is 42ms, which is small enough to doubt.
+export function shiftIndex(f, lead, n) {
+  return Math.min(n - 1, Math.max(0, f + lead));
+}