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]>

docs/CURRENT-STATE.md

@@ -3084,7 +3084,7 @@ provider draws in one colour (`PROVIDER_COLORS` in
 `src/layers/streetLevel/policy.js`); 360° cones are rings and the selected
 sequence is GEV cyan. Share options: `m` (Mapillary), `r` (the other providers' switches by id,
 e.g. `google-1` for Street View), `p` (`a`/`p`/`f` panoramas), `s` (since, days). Mapillary, Google
-Street View and KartaView are registered. Without
+Street View, KartaView and Panoramax are registered. Without
 `MAPILLARY_CLIENT_TOKEN` the panel reads KEY REQUIRED and the controls are
 disabled. With it, coverage draws as z0–5 overview points from orbit and z11–14
 sequence lines below 60 km; the proxy strips the unused `image` layer from z14
@@ -3244,14 +3244,49 @@ photo that was opening is drawn on the new canvas instead. FIT / FILL hide
 for panoramas (`supportsRenderModes`). The
 caption has no place name (KartaView's is where the drive began) and links to
 the full-size photo through the image proxy, since kartaview.org's photo
-pages fail for signed-out visitors.
+pages fail for signed-out visitors. The viewer itself is `photoViewer.js`,
+shared with Panoramax: each provider hands it a `PhotoSources` table (first
+and sharper flat and panorama URLs, and the caption fields).
+
+Panoramax (`providers/panoramax/`) is keyless and browser-side: the
+meta-catalogue `api.panoramax.xyz` (CORS open) federates every instance, and
+each picture is served by the instance holding it (OpenStreetMap France,
+IGN, MapComplete…; IGN's redirect to its OVH object storage). Its chip
+(orange, `#ff6f00`, Panoramax's own) starts off. Coverage comes from its
+vector tiles (`/api/map/{z}/{x}/{y}.mvt`, an hour's HTTP cache): sequence
+lines from z14 below 1.8 km above ground, z13 to 5 km, z12 to 12 km and z11
+to 30 km (a dense z11 tile over Paris is ~1.4 MB and 3,800 sequences; each
+tile draws its 1,600 longest), and above 30 km the `grid` layer's 0.1°
+density cells, binned to at most 96×96 points a tile (a z0 tile holds
+~70,000 cells) and not counted as sequences: the panel says to zoom in. Both
+filters apply: lines carry their day and camera type, and a cell stays while
+it holds pictures of the kind asked for (cells are undated). A click on a
+line asks the core (`actions.openNearest`) for that sequence's picture
+nearest the click (`/api/search?place_position…&collections=<id>`, any
+direction: `place_fov_tolerance=180`), without framing, then any sequence's
+if it has none within 50 m; the filters go to the API (`filter=
+field_of_view=360`, `datetime=<since>/..`). Once the picture is on screen
+its sequence is highlighted and drawn as cones from the z15 tiles around it
+(their `pictures` layer, ~110 bytes a picture, against ~7.5 kB for a STAC
+item), plus a neighbour within 150 m of an edge; the cones re-centre when a
+picture opens outside them. ‹ › follow the picture's own `prev`/`next`
+links, so stepping costs one small search per picture. The viewer shows the
+2,048 px SD derivative (equirectangular for a 360° picture) and swaps in the
+HD original only when the viewer is wider than SD (flat) or needs a 4,096 px
+panorama (expanded), once, shrunk while it decodes; originals wider than
+8,192 px are never fetched. The caption names the author (not an account
+named by an email address) and the picture's licence (CC BY-SA 4.0, Licence
+Ouverte 2.0…; StreetPose `license`), and links to the picture in the
+Panoramax viewer. The MCP globe panel allows the meta-catalogue and the
+three biggest instances; a picture on another instance shows only in the
+app.
 
 Street Level has two browser gates. `npm run qa:street-level -- --url <server>`
 runs against real Mapillary and needs `MAPILLARY_CLIENT_TOKEN`. `npm run
 qa:street-level:fixtures -- --url <server>` answers every Mapillary request
 from fixtures (`providers/mapillary/coverageFixture.mjs`,
-`scripts/fixtures/street-level/`), so it needs no network and covers the whole
-photo flow. CI runs the fixture gate and `qa:panel-resize`
+`scripts/fixtures/street-level/`), and KartaView's and Panoramax's too, so it
+needs no network and covers the whole photo flow. CI runs the fixture gate and `qa:panel-resize`
 against a production build served by `vite preview` with a dummy token.
 
 Directions is a keyless front end to the routing the voice agent already