test: add real-photo fixture harness for detection regressions

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

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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+[]