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>

README.md

@@ -7,8 +7,8 @@ RDWorks (or LightBurn/Inkscape) whether your design fits on it. Built for Makers
 
 1. Photograph the piece from straight above, on a contrasting surface, with something of known length visible.
 2. Load the photo, click **Measure**, click both ends of the known length, and enter it in mm.
-3. Check the green outline. If needed, adjust the threshold, tick "darker than background",
-   click the piece, or draw the outline by hand.
+3. Check the green outline. If needed, click the piece (this also tells the app whether
+   it is darker or lighter than the background), adjust the threshold, or draw the outline by hand.
 4. Set the safety margin (default −2 mm shrinks the usable area a little).
 5. Download DXF and import it in your laser software at 1:1, in mm.