Frame removal for plates; mouth keeps every frame

Sparseness was being applied for two different reasons at once. Aesthetic
sparseness is set by the extraction rate; labour sparseness only binds on the
plate, because a human draws each one. The mouth is traced and therefore free,
and in limited animation lip sync is routinely the densest element - on 1s
while heads hold on 2s and 3s.

So: the mouth gets a key on every frame, and the frame strip is now the
editing surface for deciding which frames need their own plate drawing. All
frames start kept; delete the ones you don't want.

- strip of face-cropped thumbnails, keep/drop per frame, keyboard driven
- worksheet panel lists the drawings needed and the range each one holds
- Suggest runs error-tolerance decimation on head pose as a starting point
- export writes sparse plate keys + dense mouth keys, with a hold manifest
- smoothContours: bounded exception to "never smooth the contour", which held
  only while keys were sparse enough to reject detector noise by sampling
- averages are now a RADIUS in frames: 0 is off, 1 is +-1
- GPU delegate falls back to CPU instead of failing
- #synth / #frames autorun for headless smoke tests

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

js/selftest.js

@@ -91,7 +91,7 @@ export function run() {
      `s=${got.s.toFixed(6)} th=${got.theta.toFixed(6)}`);
 
   // stabilisation: head motion out, mouth motion kept
-  const stab = stabilize(dense, 1);
+  const stab = stabilize(dense, 0);
   const rawSpread = spreadX(dense, 133);
   const stabSpread = (() => {
     const xs = stab.eyes.map((e) => e[0].x);