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

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

src/lib/opencv-loader.ts

@@ -0,0 +1,18 @@
+export type CV = typeof import('@techstark/opencv-js');
+
+let loading: Promise<{ cv: CV }> | null = null;
+
+// Wrapped in an object: the cv module is a thenable, so resolving with it directly never settles.
+export function loadOpenCV(): Promise<{ cv: CV }> {
+  loading ??= (async () => {
+    const mod = (await import('@techstark/opencv-js')) as unknown as { default: CV };
+    const cv = mod.default;
+    if (!cv.Mat) {
+      await new Promise<void>((resolve) => {
+        (cv as unknown as { onRuntimeInitialized: () => void }).onRuntimeInitialized = () => resolve();
+      });
+    }
+    return { cv };
+  })();
+  return loading;
+}