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>

README.md

@@ -62,6 +62,31 @@ is hand-drawn head plates, which this tool does not yet do.
 | closed-mouth cut | Aperture below which the mouth interior is emitted as `hidden`. |
 | suggest tolerance | Max head movement before a new plate drawing is required. Affects **Suggest** only. |
 
+## The plate is reference, not art
+
+The plate layer has several representations because its job changes. Cycle with
+<kbd>B</kbd>:
+
+| Mode | For |
+| --- | --- |
+| photo dim / photo | **Drawing over.** The source frame, stabilised. |
+| posterized | The footage quantised into the ramp — a look test asked of the source rather than of a drawing. |
+| oval | A flat stand-in, to judge the mouth against something. |
+| oval + photo | Checking the stand-in against the real head. |
+| none | Mouth alone. |
+
+Photo modes are **registered**: the frame is mapped into raster space through the
+same transform chain the contours go through, so the head sits still and a
+drawing traced from it is already aligned to the mouth. An unregistered underlay
+would be decorative.
+
+`MediaPipe`'s face oval is the *face* boundary — it cuts at the hairline and
+excludes hair, ears, jaw underside and neck — so as a head silhouette it is an
+egg by construction, and no landmark precision fixes that. Hence the photo.
+
+**Save frame 4x** writes the current registered composite as a 1280×800 PNG to
+draw on.
+
 ## Two kinds of sparseness
 
 Sparseness has two unrelated causes, and conflating them was the original design