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]>

CHANGELOG.md

@@ -103,7 +103,12 @@
   look, wheel to zoom), with cones along its drive and ‹ › / ← → to step
   through it. The caption links to the full-size photo (KartaView's photo
   pages fail for signed-out visitors), and the globe credits "© KartaView
-  contributors, CC BY-SA 4.0". Share links carry it as `0.m.0_0.r.kartaview-1`. Archived photos (KartaView answers 409) are skipped by the click and by ‹ ›; a keyboard and pinch look around a 360° photo.
+  contributors, CC BY-SA 4.0". Share links carry it in the provider
+  switches (`0.m.0_0.r.google-0*kartaview-1`). Archived photos (KartaView
+  answers 409) are skipped by the click and by ‹ ›. Shift+← →, ↑ ↓, + − and
+  pinch look around a 360° photo. The open photo's marker is a purple view
+  cone pointing along the drive (KartaView's own compass headings are often
+  0 or far off).
   Mapillary's line coverage and sequence cones are now shared by both
   (`lineCoverage.js`, `sequenceCones.js`).