fix(street-level): KartaView PR review — context loss mid-load, chunked reads, source switches, a11y

From the IC8 review of #1121 (2 P2, 7 P3; each with a test that fails on
the old code):

P2
- A WebGL context lost while a newer panorama loaded left a black viewer
  and a 'successful' open: the panorama now builds on the new canvas (or
  falls back to its front view).
- The streaming body read every real answer takes was tested with one
  chunk only: answers split mid-character are reassembled; the shared test
  fake now streams, so every source test runs the production path.

P3
- Lighting another source by its chip (params) now stops a running lookup
  and clears its spinner and error, as setProviderEnabled did; a restore
  that changes nothing leaves them alone.
- Panoramas decode off the main thread (ImageBitmap, RGBA upload: ~90 ms →
  ~15 ms at 4,096 px on a phone-class CPU) and are 2,048 px docked, 4,096
  once the viewer grows; a closed viewer keeps a 1×1 texture.
- The lit KARTAVIEW chip label is #8f8aff (~6:1; #635bff was 3.7:1).
- The viewer's keyboard focus ring is drawn above the photo.
- Tests: a second context loss and a drag on the new canvas; the look
  direction kept across a loss; a page's 30 s first try.
- The browser gate loses a real WebGL context in the 360° step and checks
  the 429 lookup was asked again (the fixture counter is reset first).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/streetLevel/index.test.mjs

@@ -897,3 +897,49 @@ test('KartaView through the core: its own swatch without Selected, its lines cou
     false,
   );
 });
+
+test('switching source by params (the chip path) stops a running lookup and clears its spinner and error', async (t) => {
+  const slow = slowLookups();
+  const looking = fakeProvider({ nearestImage: slow.nearestImage });
+  const other = fakeProvider({ id: 'other', pickPrefix: 'oth:' });
+  const { layer } = await enabledLayer(t, [looking, other]);
+  layer.attachViewerHost({});
+  const opening = layer.openNearest({ lat: 1, lon: 1 });
+  await settle();
+  assert.equal(layer.getUIState().street.loading, true);
+  layer.setParams({ mapillary: false, other: true });
+  assert.equal(slow.lookups[0].signal.aborted, true, 'the lookup is stopped');
+  assert.equal(
+    layer.getUIState().street.loading,
+    false,
+    'no spinner left behind',
+  );
+  // The stopped lookup answers late: nothing opens, nothing is reported.
+  slow.lookups[0].answer.resolve('img1');
+  assert.equal(await opening, false);
+  assert.deepEqual(looking.calls.open, []);
+  assert.equal(layer.getUIState().street.error, null);
+});
+
+test('switching source by params clears a failed lookup’s error; reapplying the same switches does not', async (t) => {
+  const failing = fakeProvider({
+    nearestImage: async () => {
+      throw new Error('KartaView did not answer in time — try again');
+    },
+  });
+  const { layer: second } = await enabledLayer(t, [
+    failing,
+    fakeProvider({ id: 'other', pickPrefix: 'oth:' }),
+  ]);
+  second.attachViewerHost({});
+  await second.openNearest({ lat: 1, lon: 1 });
+  assert.match(second.getUIState().street.error, /did not answer/);
+  second.setParams({ mapillary: false, other: true });
+  assert.equal(second.getUIState().street.error, null);
+  // Reapplying the same switches (a restore) leaves state alone.
+  await second.openNearest({ lat: 1, lon: 1 });
+  const kept = second.getUIState().street.error;
+  assert.match(kept, /No street-level imagery/);
+  second.setParams({ mapillary: false, other: true });
+  assert.equal(second.getUIState().street.error, kept);
+});