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/uiState.js

@@ -0,0 +1,65 @@
+/**
+ * @typedef {object} ProviderSnapshot
+ * @property {string} id
+ * @property {string} name
+ * @property {string} label
+ * @property {boolean} on
+ * @property {boolean|null} configured   null until the status call answers
+ * @property {boolean} keyRequired   no usable key: missing or rejected
+ * @property {boolean} [keyRejected] the provider refused the configured key
+ * @property {string|null} requiresKeyId
+ * @property {boolean} loading
+ * @property {number} count
+ * @property {string} hint
+ * @property {string|null} error
+ * @property {string} color   The source's one colour.
+ */
+
+/**
+ * Totals over switched-on providers. Key-gated only when every one lacks its
+ * key; `keyRejected` when that is because a key was refused.
+ * @param {Array<ProviderSnapshot>} providers
+ */
+export function summarizeCoverage(providers) {
+  const active = providers.filter((p) => p.on);
+  return {
+    count: active.reduce((sum, p) => sum + (p.count || 0), 0),
+    loading: active.some((p) => p.loading),
+    hint:
+      providers.length === 0
+        ? 'No street-level imagery providers configured'
+        : active.find((p) => p.hint)?.hint || '',
+    error: active.find((p) => p.error)?.error || null,
+    keyRequired:
+      active.length > 0 && active.every((p) => p.keyRequired === true),
+    keyRejected:
+      active.length > 0 &&
+      active.every((p) => p.keyRequired === true) &&
+      active.some((p) => p.keyRejected === true),
+  };
+}
+
+/**
+ * The panel snapshot from core state and provider snapshots.
+ * @param {{enabled: boolean, filter: object, providers: Array<ProviderSnapshot>, street: object, sequence: object}} input
+ */
+export function composeUIState({
+  enabled,
+  filter,
+  providers,
+  street,
+  sequence,
+}) {
+  const { keyRequired, keyRejected, ...coverage } =
+    summarizeCoverage(providers);
+  return {
+    enabled,
+    keyRequired,
+    keyRejected,
+    filter: { ...filter },
+    providers: providers.map((p) => ({ ...p })),
+    coverage,
+    sequence: { ...sequence },
+    street: { ...street },
+  };
+}