fix(street-level): KartaView marker and camera direction; remaining review P4s

Marker and direction (reported on the PR):
- KartaView has its own marker, a purple view cone (Street View's is
  blue), not GEV's default pin that Mapillary uses.
- The camera direction (marker, cones, FOLLOW, a panorama's centre) is
  the direction of travel to the next photo a few metres on. KartaView's
  heading is the device compass: whole drives read 0 (no compass) and
  others sit a steady ~110° off. Until the drive loads the compass stands
  in (0 = unknown), and the pose is sent again once the course is known.

Remaining P4s from the review of #1121:
- Segment chaining is linear (a per-point cursor) and capped at 20,000
  segments a tile and 5,000 points a line: a hostile tile of 60k segments
  sharing one point took 1.5 s, now tens of ms.
- Decoded tiles are dropped when KartaView is switched off.
- No network / a refused request says so and is not retried as a timeout;
  an unreadable answer says so.
- A docked flat photo skips the 2,560 px download until the viewer grows.
- The panel counts the drive's photos (countActivePhotos), not the thinned
  page of cones (2,250, not 217).
- After a GPU reset the viewer redraws the photo itself, so a lookup the
  user started is not cancelled; a failure is still reported.
- The gate's keyless page lights KartaView and checks it draws with no key;
  the fixture answers the sequence count; the line-click step checks the
  camera looks along the drive when the compass says 200°.
- README/CHANGELOG show the share-link form the app writes.

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

src/layers/streetLevel/providers/kartaview/viewer.test.mjs

@@ -168,6 +168,7 @@ test('a flat photo shows its 1280 px version, then the sharp one, with a pose fa
   });
   await viewer.mount(host);
   const { picture, canvas, prev, next } = parts(host);
+  picture.clientWidth = 1600; // expanded: the sharp version is wanted
   assert.equal(prev.disabled, true, 'nothing to step through yet');
   await viewer.open('flat1');
   assert.equal(picture.hidden, false);
@@ -621,6 +622,8 @@ test('quick presses while the next photo is still loading each step on from the
 test('a slow sharp version of an older photo never replaces the newer photo', async () => {
   const { viewer, host, shown, poses, release } = heldSetup();
   await viewer.mount(host);
+  // An expanded viewer, so the sharp version is wanted.
+  parts(host).picture.clientWidth = 1600;
   const first = viewer.open('a');
   await release('a@1280');
   await first;
@@ -904,3 +907,19 @@ test('a CORS image is decoded off the main thread into an ImageBitmap where the
     /could not be loaded/,
   );
 });
+
+test('a docked viewer keeps the 1,280 px photo; expanding it brings the 2,560 px one', async () => {
+  const { viewer, host, loads } = setup();
+  await viewer.mount(host);
+  await viewer.open('flat1');
+  for (let i = 0; i < 4; i++) await settle();
+  assert.equal(loads.length, 1, 'no 2,560 px download for a 400 px frame');
+  parts(host).picture.clientWidth = 1600;
+  viewer.resize();
+  assert.ok(await until(() => loads.length === 2));
+  assert.match(loads[1].url, /rs:fit:2560:2560/);
+  assert.equal(parts(host).picture.src, loads[1].url);
+  viewer.resize();
+  for (let i = 0; i < 4; i++) await settle();
+  assert.equal(loads.length, 2, 'once only');
+});