feat: add OpenCV loader, Otsu threshold and margin-aware mask tracing

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

tests/helpers/images.ts

@@ -0,0 +1,35 @@
+import type { Mat } from '@techstark/opencv-js';
+import type { CV } from '../../src/lib/opencv-loader';
+import type { RasterImage } from '../../src/lib/types';
+
+/** Grayscale RGBA image where `paint(x, y)` returns the brightness 0–255. */
+export function grayImage(width: number, height: number, paint: (x: number, y: number) => number): RasterImage {
+  const data = new Uint8ClampedArray(width * height * 4);
+  for (let y = 0; y < height; y++) {
+    for (let x = 0; x < width; x++) {
+      const i = (y * width + x) * 4;
+      const v = paint(x, y);
+      data[i] = data[i + 1] = data[i + 2] = v;
+      data[i + 3] = 255;
+    }
+  }
+  return { width, height, data };
+}
+
+/** CV_8UC1 mask with 255 where `inside(x, y)`. Caller must delete it. */
+export function maskFrom(cv: CV, width: number, height: number, inside: (x: number, y: number) => boolean): Mat {
+  const mask = cv.Mat.zeros(height, width, cv.CV_8UC1) as Mat;
+  const data = mask.data;
+  for (let y = 0; y < height; y++) {
+    for (let x = 0; x < width; x++) {
+      if (inside(x, y)) data[y * width + x] = 255;
+    }
+  }
+  return mask;
+}
+
+export const inRect = (x: number, y: number, x0: number, y0: number, x1: number, y1: number) =>
+  x >= x0 && x < x1 && y >= y0 && y < y1;
+
+export const inCircle = (x: number, y: number, cx: number, cy: number, r: number) =>
+  (x - cx) ** 2 + (y - cy) ** 2 <= r * r;