Propose black and white points from the scan

Levels shipped at 0–255 unless someone moved the sliders, and Bicycle
Race showed what that costs. Its ink is grey, not black — a scanned
engraving, ink at 2–95, paper at 163–255 — and with alpha = 255 − luminance
that greyness becomes transparency. No pixel in the exported bundle was
even fully opaque, and the downscale to the song's width blended every
stroke edge further. Nothing downstream can rescue it.

So detection proposes levels too, like it proposes cuts and skew.
Notation is two-tone, which makes Otsu's split the measurement wanted;
the points sit halfway from it to each end of the range, so the ramp
between them survives as antialiasing rather than going jagged. A page
already scanned bilevel has no interior split — Otsu degenerates to 0 —
and is left alone. Per page, with the median becoming the song's, so a
near-blank page cannot set them.

docs/guide.md

@@ -44,10 +44,11 @@ off.
    one back.
 4. **Set the content rectangle.** Drag the blue edges so they hold the music and
    nothing else. This is the horizontal crop for every slice on the page.
-5. **Set black and white points.** Pull the *White point* down until the paper
-   goes pure white and its texture disappears; pull *Black point* up until the
-   notes are solid black rather than grey mush. Scans need this; clean digital
-   PDFs usually don't.
+5. **Check black and white points.** These arrive proposed from the scan, like
+   the cuts do. If the paper still shows texture, pull *White point* down; if the
+   notes look grey rather than solid, pull *Black point* up. Getting this wrong
+   is the one mistake you cannot see until the bundle is on the tablet — grey ink
+   becomes half-transparent ink, and nothing downstream can rescue it.
 
 Page Up / Page Down move between pages. Levels carry over from the previous
 page, so a consistent scan only needs setting once.