Registered photo underlay as the plate reference

The generated face oval was never going to be good enough to draw from:
MediaPipe's face oval is the FACE boundary, cut at the hairline and excluding
hair, ears, jaw and neck, so it is an egg by construction. Segmentation would
give a real head outline but costs a 16MB model and per-frame inference for a
shape that gets replaced by a drawing anyway.

So the plate layer becomes switchable, and the useful modes are photographic:
the source frame mapped into raster space through the same transform chain the
contours go through. Registration is the whole point - the head sits still and
a drawing traced from the underlay is already aligned to the mouth. An
unregistered underlay would be decoration.

- underlay.js: pixel->raster affine (a general affine, since MediaPipe
  normalises x by width and y by height), registered draw, palette posterise
- plate modes: photo / photo dim / posterized / oval / oval+photo / none, B cycles
- worksheet cells are registered composites rather than raw crops
- Save frame 4x writes a 1280x800 PNG to draw on
- selftest: FACE_OVAL simplicity, which was never asserted; a wrong ordering
  there reads as a lumpy plate rather than an obvious bowtie

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

.gitignore

@@ -1,3 +1,5 @@
 frames/
 *.task
 *.take
+
+*.tflite