Merge origin/main: keep the provider registry, slim it

main grew the Street Level provider registry while this branch removed it
(#982–#985: injected providers, an empty composition, the `r` switch field).
The registry, params, uiState and pick router come back as main has them and
the slimmed layer sits behind the provider contract:

- The core (index.js) drives providers as on main, without surface modes,
  ground casting, FOLLOW, prewarm and nearest-image lookups. A provider's key
  status is asked with a 10 s timeout and asked again on a switch-on while no
  key is known.
- Mapillary is mapillaryProvider.js over the slim coverage, sequences and
  viewer modules, which read the filter and the active flag from the provider
  context.
- The viewer host mounts one provider adapter at a time, no prewarm.
- The panel has a chip per provider again (the pill switches the layer);
  no legend, EXPAND stays the Fullscreen API.
- The contract drops what no provider implements: groundCaster, meshSampler,
  getSurface/setSurface, nearestImage, prewarm, reportError, the legend and
  surface fields of the UI snapshot.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/layers/streetLevel/policy.js

@@ -1,8 +1,18 @@
-/** Identity and tuning for the Street Level layer and its Mapillary imagery. */
+/** Identity and tuning for the Street Level layer and its Mapillary provider. */
 export const STREET_LEVEL_LAYER_ID = 'street-level';
+export const MAPILLARY_PROVIDER_ID = 'mapillary';
+export const MAPILLARY_NAME = 'Mapillary';
+export const MAPILLARY_LABEL = 'MAPILLARY';
 export const MAPILLARY_KEY_ID = 'mapillary';
 export const MAPILLARY_GRAPH_HOST = 'https://graph.mapillary.com';
 
+/** What the provider needs from its source (source.js or a stand-in). */
+export const MAPILLARY_SOURCE_METHODS = Object.freeze([
+  'getStatus',
+  'getTile',
+  'getSequenceImages',
+]);
+
 /** Pick id of the viewer position marker. */
 export const POSITION_PICK_ID = 'sl:pos';