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/streetLevelMarkup.test.mjs

@@ -46,6 +46,9 @@ test('the key gate holds every filter, with the error line outside it', () => {
   assert.match(gate, /data-sl-pano="all"/);
   assert.match(gate, /id="sl-since"/);
   assert.ok(html.indexOf('id="sl-error"') < start);
+  // The chips stay outside the gate, so their key tooltip is reachable.
+  assert.match(html, /<div id="sl-provider-chips" class="sl-chips"><\/div>/);
+  assert.doesNotMatch(gate, /sl-provider-chips/);
   assert.match(html, /id="sl-error"[^>]*role="alert"/);
 });