test: add real-photo fixture harness for detection regressions

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

tests/fixtures.test.ts

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+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import jpeg from 'jpeg-js';
+import { beforeAll, describe, expect, it } from 'vitest';
+import { detect, otsuThreshold } from '../src/lib/detect';
+import { prepareForExport, shapeSize } from '../src/lib/geometry';
+import { loadOpenCV, type CV } from '../src/lib/opencv-loader';
+import type { Point, RasterImage } from '../src/lib/types';
+
+type FixtureCase = {
+  file: string;
+  threshold: number | null;
+  invert: boolean;
+  seed: Point | null;
+  pxPerMm: number;
+  expected: { widthMm: number; heightMm: number; holes: number };
+  toleranceMm: number;
+};
+
+const dir = fileURLToPath(new URL('./fixtures/', import.meta.url));
+const cases: FixtureCase[] = JSON.parse(readFileSync(dir + 'manifest.json', 'utf8'));
+
+function readJpeg(path: string): RasterImage {
+  const decoded = jpeg.decode(readFileSync(path), { useTArray: true, formatAsRGBA: true });
+  return { width: decoded.width, height: decoded.height, data: new Uint8ClampedArray(decoded.data) };
+}
+
+describe.skipIf(cases.length === 0)('detect on real photos', () => {
+  let cv: CV;
+  beforeAll(async () => {
+    ({ cv } = await loadOpenCV());
+  });
+
+  it.each(cases)('$file', (c) => {
+    const image = readJpeg(dir + c.file);
+    const threshold = c.threshold ?? otsuThreshold(cv, image);
+    const shape = detect(cv, image, { threshold, invert: c.invert, seed: c.seed, marginPx: 0 });
+    expect(shape).not.toBeNull();
+    const mm = prepareForExport(shape!, c.pxPerMm)!;
+    const size = shapeSize(mm);
+    expect(Math.abs(size.width - c.expected.widthMm)).toBeLessThanOrEqual(c.toleranceMm);
+    expect(Math.abs(size.height - c.expected.heightMm)).toBeLessThanOrEqual(c.toleranceMm);
+    expect(mm.holes).toHaveLength(c.expected.holes);
+  });
+});

tests/fixtures/README.md

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+# Real photo fixtures
+
+Regression photos for `detect()`. Each entry in `manifest.json` is checked by `tests/fixtures.test.ts`.
+
+To add one:
+
+1. Take a top-down photo of a scrap piece with something of known length visible.
+2. Rotate it upright and resize it so the long side is at most 2000 px. Save as JPEG.
+   EXIF orientation is ignored by the test decoder.
+3. Measure the real piece's bounding box in mm.
+4. Open the photo in the app, set the scale, and note px/mm, the threshold you used, and,
+   if needed, a seed point (shown in image px).
+5. Add an entry:
+
+```json
+{
+  "file": "plywood-1.jpg",
+  "threshold": null,
+  "invert": false,
+  "seed": null,
+  "pxPerMm": 3.21,
+  "expected": { "widthMm": 412, "heightMm": 287, "holes": 2 },
+  "toleranceMm": 8
+}
+```
+
+`threshold: null` means Otsu's automatic value. `seed` is `{ "x": ..., "y": ... }` or `null`.

tests/fixtures/manifest.json

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+[]