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/manifest.json

@@ -1 +1,10 @@
-[]
+[
+  {
+    "file": "mdf_floor.jpeg",
+    "threshold": null,
+    "seed": null,
+    "pxPerMm": 1,
+    "expected": { "widthMm": 1714, "heightMm": 675 },
+    "toleranceMm": 60
+  }
+]