feat: correct uneven lighting and pick polarity automatically

Fit the background brightness from the photo border and subtract it, so
side-lit photos threshold cleanly. The clicked pixel (or, without a click,
the border) decides whether the piece is darker or lighter, replacing the
invert checkbox. Adds the MDF-on-wood-floor photo as a regression fixture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/fixtures/README.md

@@ -16,7 +16,6 @@ To add one:
 {
   "file": "plywood-1.jpg",
   "threshold": null,
-  "invert": false,
   "seed": null,
   "pxPerMm": 3.21,
   "expected": { "widthMm": 412, "heightMm": 287, "holes": 2 },
@@ -24,4 +23,5 @@ To add one:
 }
 ```
 
-`threshold: null` means Otsu's automatic value. `seed` is `{ "x": ..., "y": ... }` or `null`.
+`threshold: null` means Otsu's automatic value. `holes` is optional; leave it out when
+background texture creates noise holes. Without a known scale, use `pxPerMm: 1` and sizes in px. `seed` is `{ "x": ..., "y": ... }` or `null`.