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/ui/templates/layer-panels.html

@@ -107,8 +107,13 @@
             </span>
           </div>
         </div>
+        <div class="sl-providers" role="group" aria-label="Imagery providers">
+          <span class="cctv-summary-label">PROVIDERS</span>
+          <div id="sl-provider-chips" class="sl-chips"></div>
+        </div>
         <div id="sl-error" class="sl-error" role="alert" hidden><span aria-hidden="true">⚠</span> <span id="sl-error-text"></span></div>
-        <!-- A missing key gates the filters; the line above says how to add it. -->
+        <!-- A missing key gates the filters only: the chips above stay
+             focusable so their tooltip can say how to add the key. -->
         <fieldset id="sl-controls" class="sl-controls">
           <div class="sl-filters" role="group" aria-label="Imagery filters">
             <span class="cctv-summary-label">IMAGERY</span>