feat(street-level): Panoramax as a fourth, keyless imagery provider

Panoramax, the open federated street-level commons, registers beside
Mapillary, Google Street View and KartaView. Its meta-catalogue
(api.panoramax.xyz) is keyless and CORS-open, so it runs in the browser:

- coverage from its vector tiles: sequence lines at z11–14 below 30 km
  (longest first under the per-tile cap), and above that the grid layer's
  density cells as binned points, not counted as sequences;
- a click on a line opens that sequence's picture nearest the click
  (STAC search, any direction), without moving the camera, then any
  sequence's if it has none within 50 m; the 360° and since filters go to
  the lines and to the API;
- cones from the z15 tiles' pictures layer around the open picture
  (~110 bytes a picture, against ~7.5 kB for a STAC item), re-centred when
  a picture opens outside them; ‹ › follow the picture's prev/next links;
- the caption names the author (not an account named by an email
  address) and the picture's licence (StreetPose `license`), and links to
  the picture in the Panoramax viewer; the globe credits Panoramax.

KartaView's viewer becomes the shared photoViewer.js (flat <img>, 360° on
panorama.js, ‹ ›, keyboard, pinch, context loss), driven by a per-provider
PhotoSources table; the request limiter moves to limiter.js. Panoramax shows
the 2,048 px SD derivative and fetches the HD original only when the viewer
is wide enough, once, shrunk while it decodes (never above 8,192 px).

Orange #ff6f00 (Panoramax's own), off by default, share option
0.m.0_0.r.panoramax-1. The MCP panel allows the meta-catalogue and the
biggest instances. Tests: policy, decode, source, viewer, provider, and a
Panoramax fixture in the hermetic browser gate.

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

CHANGELOG.md

@@ -95,6 +95,19 @@
 
 ## [Unreleased]
 
+- Street Level: Panoramax is the fourth provider, keyless. Light its chip
+  (orange; off by default, one provider at a time): from orbit dots show
+  where the federated commons has pictures, and below about 30 km its
+  sequences draw as lines from `api.panoramax.xyz` vector tiles. A click on a
+  line opens that sequence's picture nearest the click without moving the
+  camera, flat or 360°, with cones along the sequence around it and ‹ › /
+  ← → following the picture's own links. The 360° and since filters apply,
+  in the lines and in the lookup. The caption names each picture's author
+  and licence (CC BY-SA 4.0, Licence Ouverte 2.0…) and links to it in the
+  Panoramax viewer; HD originals load only for an expanded viewer, once.
+  Share links carry it as `0.m.0_0.r.panoramax-1`. KartaView's photo viewer
+  is now shared (`photoViewer.js`, `panorama.js`).
+
 - Street Level: KartaView is the third provider, keyless. Light its chip
   (purple; off by default, one provider at a time) and zoom in below about
   6 km: the lines are the streets KartaView has photos of (z13–14 tiles from