feat: run manual polygons through the shared margin pipeline

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

src/lib/margin.ts

@@ -0,0 +1,31 @@
+import type { Mat } from '@techstark/opencv-js';
+import { traceMask } from './detect';
+import type { CV } from './opencv-loader';
+import type { Point, Shape } from './types';
+
+export function shapeFromPolygons(cv: CV, width: number, height: number, polygons: Shape, marginPx: number): Shape | null {
+  if (polygons.outer.length < 3) return null;
+  const mask = cv.Mat.zeros(height, width, cv.CV_8UC1) as Mat;
+  try {
+    fill(cv, mask, [polygons.outer], 255);
+    fill(cv, mask, polygons.holes.filter((h) => h.length >= 3), 0);
+    return traceMask(cv, mask, marginPx);
+  } finally {
+    mask.delete();
+  }
+}
+
+function fill(cv: CV, mask: Mat, rings: Point[][], value: number) {
+  if (rings.length === 0) return;
+  const mats = rings.map((ring) =>
+    cv.matFromArray(ring.length, 1, cv.CV_32SC2, ring.flatMap((p) => [Math.round(p.x), Math.round(p.y)])),
+  );
+  const vector = new cv.MatVector();
+  try {
+    mats.forEach((m) => vector.push_back(m));
+    cv.fillPoly(mask, vector, new cv.Scalar(value));
+  } finally {
+    vector.delete();
+    mats.forEach((m) => m.delete());
+  }
+}