feat(proxy): add offset param to inset or outset traced shapes

Trace results can now be inset (negative) or outset (positive) by a
pixel amount. Offset the outline and holes in opposite directions with
clipper-lib's miter joins, then recombine with a boolean difference so
holes grow on inset and shrink on outset. Collapsed shapes return an
empty outline; a split outline keeps the largest ring. Document the
parameter and the new dependency.

apps/proxy/src/routes/trace.test.ts

@@ -100,4 +100,48 @@ describe('trace route', () => {
     expect(result.mode).toBe('bw');
     expect(result.shape.outline.length).toBeGreaterThan(3);
   });
+
+  it('applies an inset offset to the shape', async () => {
+    stubFetch(await makePng());
+    const res = await trace.request(
+      '/?url=https%3A%2F%2Fexample.com%2Fa.png&offset=-2',
+    );
+    expect(res.status).toBe(200);
+    const result = deserialize(new Uint8Array(await res.arrayBuffer()));
+    expect(result.offset).toBe(-2);
+    expect(result.shape.outline.length).toBeGreaterThan(3);
+    // An inset of 2px on a 20px circle must be visibly smaller.
+    const extent = (pts: number[][]) => [
+      Math.min(...pts.map((p) => p[0]!)),
+      Math.max(...pts.map((p) => p[0]!)),
+    ];
+    const [minX, maxX] = extent(result.shape.outline);
+    expect((maxX ?? 0) - (minX ?? 0)).toBeLessThan(20);
+  });
+
+  it('applies an outset offset to the shape', async () => {
+    stubFetch(await makePng());
+    const res = await trace.request(
+      '/?url=https%3A%2F%2Fexample.com%2Fa.png&offset=2',
+    );
+    expect(res.status).toBe(200);
+    const result = deserialize(new Uint8Array(await res.arrayBuffer()));
+    expect(result.offset).toBe(2);
+    expect(result.shape.outline.length).toBeGreaterThan(3);
+  });
+
+  it('rejects a non-numeric offset', async () => {
+    stubFetch(await makePng());
+    const res = await trace.request(
+      '/?url=https%3A%2F%2Fexample.com%2Fa.png&offset=abc',
+    );
+    expect(res.status).toBe(400);
+  });
+
+  it('omits offset when not requested', async () => {
+    stubFetch(await makePng());
+    const res = await trace.request('/?url=https%3A%2F%2Fexample.com%2Fa.png');
+    const result = deserialize(new Uint8Array(await res.arrayBuffer()));
+    expect(result.offset).toBeUndefined();
+  });
 });