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/styles/street-level.css

@@ -110,6 +110,56 @@
   pointer-events: none;
 }
 
+/* ── Providers ───────────────────────────────────────────────────────────── */
+.sl-providers {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  flex-wrap: wrap;
+}
+
+.sl-chips {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px;
+}
+
+/* `--chip-color` comes from the provider. */
+.sl-chips .data-toggle-chip {
+  display: inline-flex;
+  align-items: center;
+  gap: 5px;
+}
+
+.sl-chips .data-toggle-chip::before {
+  content: '';
+  width: 7px;
+  height: 7px;
+  border-radius: 50%;
+  background: var(--chip-color, currentColor);
+  opacity: 0.45;
+  flex: none;
+}
+
+.sl-chips .data-toggle-chip.active {
+  color: var(--chip-color, var(--accent));
+  border-color: var(--chip-color, var(--accent));
+  background: color-mix(
+    in srgb,
+    var(--chip-color, var(--accent)) 14%,
+    transparent
+  );
+}
+
+.sl-chips .data-toggle-chip.active::before {
+  opacity: 1;
+}
+
+.sl-chips .data-toggle-chip.chip-error {
+  color: #ffb300;
+  border-color: rgba(255, 179, 0, 0.55);
+}
+
 /* ── Errors ──────────────────────────────────────────────────────────────── */
 .sl-error {
   font-family: var(--font-sans);