fix(street-level): review round — safer docking, mount retry, provider fallthrough

- Only a user's own collapse docks a floating window; cockpit entry,
  restores and the Cyber accordion no longer dock it or delete its saved
  place, and the accordion leaves floating windows alone.
- The viewer host records an adapter as active only after its mount
  succeeds, so a library that failed to load once is retried; concurrent
  opens share one mount, and unmounting mid-mount leaves nothing active.
- openNearest tries every active provider; one provider's error (no key,
  offline) no longer hides the others' imagery.
- The SINCE readout shows its cut-off date as soon as the committed value
  matches the thumb, even while the slider keeps focus.
- A floating window that was only dragged is bounded to the viewport and
  scrolls instead of running off-screen.
- getStats summarises coverage (summarizeCoverage) instead of building a
  full UI snapshot every second; disable() uses credits.hideAll(); smaller
  cleanups in the viewer host and sequence snapshot.
- Unit tests: panel docking and header double press, viewer-host mount
  retry/sharing/cancel, coverage summary.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/streetLevel/uiState.test.mjs

@@ -1,6 +1,6 @@
 import assert from 'node:assert/strict';
 import test from 'node:test';
-import { composeUIState } from './uiState.js';
+import { composeUIState, summarizeCoverage } from './uiState.js';
 
 const provider = (overrides = {}) => ({
   id: 'mapillary',
@@ -102,3 +102,21 @@ test('hint and error come from the first active provider that has one', () => {
   assert.deepEqual(ui.filter, base.filter);
   assert.notEqual(ui.filter, base.filter, 'snapshot copies the filter');
 });
+
+test('summarizeCoverage is what getStats reports, without building a snapshot', () => {
+  assert.deepEqual(
+    summarizeCoverage([
+      provider({ count: 4, hint: 'Look down' }),
+      provider({ id: 'b', name: 'B', count: 6, loading: true, error: 'boom' }),
+      provider({ id: 'c', name: 'C', count: 99, on: false, keyRequired: true }),
+    ]),
+    {
+      count: 10,
+      loading: true,
+      hint: 'Look down',
+      error: 'boom',
+      keyRequired: false,
+    },
+  );
+  assert.equal(summarizeCoverage([]).keyRequired, false);
+});