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]>

scripts/fixtures/street-level/kartaView.mjs

@@ -66,7 +66,8 @@ function record(photo) {
     lng: String(photo.lon),
     matchLat: String(photo.lat),
     matchLng: String(photo.lon),
-    heading: '0',
+    // A dashcam phone's compass, disturbed by the car: the drive runs north.
+    heading: '200',
     fieldOfView: photo.isPano ? '360' : '0',
     projection: photo.isPano ? 'SPHERE' : 'PLANE',
     shotDate: SHOT,
@@ -117,6 +118,7 @@ export function describeKartaViewCall(method, url) {
     .replace(/\/tiles\/\d+\/\d+\/\d+\.json$/, '/tiles/{x}/{y}/{z}.json')
     .replace(/\/photo\/\d+$/, '/photo/{id}')
     .replace(/\/sequence\/\d+\/photos$/, '/sequence/{id}/photos')
+    .replace(/\/sequence\/\d+$/, '/sequence/{id}')
     .replace(/^\/files\/photo\/.*$/, '/files/photo/{path}')
     .replace(
       /^\/pr:sharp\/rs:fit:\d+:\d+\/.*$/,
@@ -202,6 +204,18 @@ export function answerKartaViewRequest({ method, url }) {
       ? { ...ok({ data: record(photo) }), known: true }
       : { ...raw(200, { status: { apiCode: 601 }, result: {} }), known: true };
   }
+  const sequence = path.match(/^\/2\.0\/sequence\/(\d+)$/);
+  if (sequence)
+    return {
+      ...ok({
+        data: {
+          id: sequence[1],
+          countActivePhotos:
+            sequence[1] === KV_SEQUENCE_ID ? String(photos.length) : '0',
+        },
+      }),
+      known: true,
+    };
   const page = path.match(/^\/2\.0\/sequence\/(\d+)\/photos$/);
   if (page) {
     const q = address.searchParams;