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/selftest.js

@@ -9,7 +9,7 @@
 
 import { LIPS_OUTER, LIPS_INNER, FACE_OVAL, RIGID, subsampleSlots, subsampleRing } from './landmarks.js';
 import { fitSimilarity, applySim, procrustesMean, smoothTransforms } from './mathutil.js';
-import { stabilize, toRasterRing, selectKeys, activeKey } from './pipeline.js';
+import { stabilize, toRasterRing, selectKeys, activeKey, shiftIndex } from './pipeline.js';
 import { IndexedRaster, hexToRgb } from './raster.js';
 import { writeTake } from './take.js';
 import { otsuForTest, scaleRing } from './interior.js';
@@ -206,6 +206,31 @@ export function run() {
   ok('activeKey holds between keys',
      activeKey(sel.keys, sel.keys[1].f - 1).f === sel.keys[0].f);
 
+  // mouth lead: a shift that "feels like it does nothing" is indistinguishable
+  // from one that does nothing, so assert the arithmetic directly.
+  ok('lead 0 is identity', [0, 5, 71].every((f) => shiftIndex(f, 0, 72) === f));
+  ok('positive lead moves the source frame forward', shiftIndex(10, 2, 72) === 12);
+  ok('negative lead moves it back', shiftIndex(10, -3, 72) === 7);
+  ok('lead clamps at the start', shiftIndex(1, -6, 72) === 0);
+  ok('lead clamps at the end', shiftIndex(70, 6, 72) === 71);
+  {
+    // ...and that it selects different POSES, not merely different indices.
+    // Checked across the whole track rather than at one pair: synthetic poses
+    // hold for nine-frame beats, so any single pair can legitimately be
+    // identical while the shift works perfectly.
+    const N = shapes.length;
+    let moved = 0, total = 0;
+    for (let f = 0; f < N; f++) {
+      const a = shapes[shiftIndex(f, 0, N)], b = shapes[shiftIndex(f, 3, N)];
+      let d = 0;
+      for (let i = 0; i < a.length; i++) d += Math.hypot(a[i].x - b[i].x, a[i].y - b[i].y);
+      total++;
+      if (d / a.length > 0.5) moved++;
+    }
+    ok('a lead of 3 changes the pose on a good share of frames', moved / total > 0.2,
+       `${moved}/${total} frames differ`);
+  }
+
   // rasteriser: indexed, hard-edged, no blending
   const r = new IndexedRaster(64, 48);
   r.clear(0);