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/detect.test.ts

@@ -1,5 +1,5 @@
 import { beforeAll, describe, expect, it } from 'vitest';
-import { detect } from '../src/lib/detect';
+import { detect, otsuThreshold } from '../src/lib/detect';
 import { bounds } from '../src/lib/geometry';
 import { loadOpenCV, type CV } from '../src/lib/opencv-loader';
 import type { DetectParams } from '../src/lib/types';
@@ -11,7 +11,7 @@ beforeAll(async () => {
 });
 
 const params = (overrides: Partial<DetectParams> = {}): DetectParams => ({
-  threshold: 128, invert: false, seed: null, marginPx: 0, minHolePx: 0, ...overrides,
+  threshold: 128, seed: null, marginPx: 0, minHolePx: 0, ...overrides,
 });
 
 /** Light plate x 40..159, y 30..129 with hole r=15 at (100,80), plus a small light blob, on dark ground. */
@@ -31,24 +31,35 @@ describe('detect', () => {
     expect(shape.holes).toHaveLength(1);
   });
 
-  it('handles a dark piece on a light background with invert', () => {
+  it('finds a dark piece on a light background without any setting', () => {
     const inverted = grayImage(200, 160, (x, y) => 250 - (inRect(x, y, 40, 30, 160, 130) ? 220 : 30));
-    expect(detect(cv, inverted, params())).toBeNull();
-    const b = bounds(detect(cv, inverted, params({ invert: true }))!.outer);
+    const b = bounds(detect(cv, inverted, params())!.outer);
     near(b.minX, 40); near(b.maxX, 159);
   });
 
+  it('takes the polarity from the clicked pixel', () => {
+    const inverted = grayImage(200, 160, (x, y) => 250 - (inRect(x, y, 40, 30, 160, 130) ? 220 : 30));
+    const b = bounds(detect(cv, inverted, params({ seed: { x: 100, y: 80 } }))!.outer);
+    near(b.minX, 40); near(b.maxX, 159);
+  });
+
+  it('corrects uneven lighting across the photo', () => {
+    // Light from the right: the floor ramps 120 → 220 and the piece is 50 darker than the floor around it.
+    const sideLit = grayImage(400, 300, (x, y) => 120 + (100 * x) / 400 - (inRect(x, y, 100, 75, 300, 225) ? 50 : 0));
+    const shape = detect(cv, sideLit, params({ threshold: otsuThreshold(cv, sideLit) }))!;
+    const b = bounds(shape.outer);
+    near(b.minX, 100); near(b.maxX, 299); near(b.minY, 75); near(b.maxY, 224);
+  });
+
   it('selects the clicked piece even when another piece is larger', () => {
     const shape = detect(cv, plateScene, params({ seed: { x: 177, y: 146 } }))!;
     const b = bounds(shape.outer);
     near(b.minX, 170); near(b.maxX, 184);
   });
 
-  it('falls back to the automatic choice when the seed is on background or in a hole', () => {
-    for (const seed of [{ x: 5, y: 5 }, { x: 100, y: 80 }]) {
-      const b = bounds(detect(cv, plateScene, params({ seed }))!.outer);
-      near(b.minX, 40);
-    }
+  it('falls back to the automatic choice when the click is on the background', () => {
+    const b = bounds(detect(cv, plateScene, params({ seed: { x: 5, y: 5 } }))!.outer);
+    near(b.minX, 40);
   });
 
   it('ignores a seed outside the image', () => {