feat: read current camera frames through the CCTV source

Add getFrame(camera) to the CCTV source. It reads one frame through the
registered frame endpoint the layer already uses and returns its content
type and bytes, so code outside the layer can show a camera's view.

src/layers/cctv/source.test.mjs

@@ -148,3 +148,28 @@ test('camera construction is inert and destruction cancels a pending catalog and
   assert.equal(a.getStats().count, 0);
   assert.equal(b.getStats().count, 0);
 });
+
+test('frames are read through the registered frame endpoint', async () => {
+  const png = Uint8Array.from([0x89, 0x50, 0x4e, 0x47]);
+  const requested = [];
+  const source = createCctvSource({
+    fetchImpl: async (url, init) => {
+      requested.push({ url, cache: init.cache });
+      return new Response(png, {
+        headers: { 'Content-Type': 'Image/PNG; charset=binary' },
+      });
+    },
+  });
+  const frame = await source.getFrame(camera);
+  assert.equal(frame.contentType, 'image/png');
+  assert.deepEqual([...frame.bytes], [...png]);
+  assert.equal(
+    requested[0].url.split('?')[0],
+    '/api/cctv/frame/pack%2Fcamera%20%3Fx',
+  );
+  assert.equal(requested[0].cache, 'no-store');
+  const failing = createCctvSource({
+    fetchImpl: async () => new Response(null, { status: 502 }),
+  });
+  await assert.rejects(failing.getFrame(camera), /Camera frame HTTP 502/);
+});