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.

tests/test_detect.py

@@ -14,7 +14,12 @@ import numpy as np
 
 sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
 
-from noteman_slicer.detect import deskew, deskew_angle, detect_page  # noqa: E402
+from noteman_slicer.detect import (  # noqa: E402
+    deskew,
+    deskew_angle,
+    detect_page,
+    ink_levels,
+)
 
 W, H = 1000, 1400
 STAFF_GAP = 15  # → staff height 60, so expansion reaches 90px past a bracket
@@ -71,6 +76,15 @@ def main() -> int:
         found = deskew_angle(deskew(page, angle))
         assert abs(found + angle) <= 0.15, f"skew {angle}: got {found}"
 
+    # Levels are proposed too. A grey scan left at 0–255 ships its wash to the
+    # tablet, and the downscale to the song's width only blends it further.
+    grey = np.full((H, W), 210, np.uint8)  # paper, not white
+    grey[200:400, 100:900] = 70  # ink, not black
+    black, white = ink_levels(grey)
+    assert black < 70 < white < 210, (black, white)
+    # A page already bilevel has nothing between ink and paper to stretch.
+    assert ink_levels(_page()) == (0, 255)
+
     # A scanner's edge line runs the whole height of the sheet. Being taller
     # than every bracket it used to win each overlap and swallow the page into
     # one system — Olukainen juomukainen, where five pages of six came out as a