fix: apply the safety margin with a distance transform

An elliptical erode kernel costs radius² per pixel. While typing a length,
an intermediate value like "3" mm made the margin hundreds of pixels and
froze the page for minutes.

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

tests/trace-mask.test.ts

@@ -85,6 +85,17 @@ describe('traceMask', () => {
   });
 });
 
+describe('traceMask performance', () => {
+  it('handles a huge margin quickly (e.g. while a length is half-typed)', () => {
+    const m = maskFrom(cv, 2000, 1500, (x, y) => inRect(x, y, 200, 200, 1800, 1300));
+    mats.push(m);
+    const start = performance.now();
+    const shape = traceMask(cv, m, -300)!;
+    expect(performance.now() - start).toBeLessThan(1500);
+    expect(Math.abs(bounds(shape.outer).minX - 500)).toBeLessThanOrEqual(2);
+  });
+});
+
 describe('otsuThreshold', () => {
   it('lands between background and piece brightness', () => {
     const image = grayImage(200, 160, (x, y) => (inRect(x, y, 40, 30, 160, 130) ? 220 : 30));