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/ui/styles/street-level.css

@@ -426,6 +426,14 @@ button.sl-status:focus-visible {
 }
 
 /* ── Floating window ─────────────────────────────────────────────────────── */
+/* A window that was only dragged keeps its natural height; bound it to the
+   viewport so nothing runs off-screen (the rail rule's max-height: 100% does
+   not bind an auto-height parent). Id-qualified to outrank that rule. */
+#right-context-rail > #street-level-panel.panel-floating > .street-level-panel-inner {
+  max-height: calc(100vh - 12px);
+  overflow-y: auto;
+}
+
 /* A resized window (PanelPositionControls writes an inline height) hands the
    spare height to the viewer, so the image never needs scrolling to. */
 .panel-floating[style*='height'] > .street-level-panel-inner {