fix(street-level): viewer lifecycle, hover cost, keyboard focus, panel lift

From the IC8 review of #768 (P2s), each with a test that fails before:

- Switching the layer off and on during the first photo download no longer
  leaves the viewer stuck on "not mounted": an outdated mount only unmounts
  an adapter nobody else holds and subscribes poses only once current, and
  the MapillaryJS adapter never builds a viewer after it was unmounted.
- Hover picks at most every 120 ms (the CCTV convention), not while the
  camera moves or a button is held, and not after the layer went off.
- Esc shrinks the expanded viewer wherever focus is (document capture), so
  the map keeps its selection, and Tab from outside returns into it.
  Closing a photo moves focus to the panel instead of dropping it on body.
- Portable panels lift only after a mouse or pen moves past the drag
  threshold: a click on a docked panel's edge or a touch swipe on its
  header no longer turns it into a floating window, and pointercancel
  saves nothing and puts the panel back.

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

src/layers/streetLevel/viewerHost.js

@@ -76,19 +76,19 @@ export function createViewerHost({ state, parts }) {
     }
     const adapter = entry.instance.viewer;
     const promise = (async () => {
-      const unsubscribe = adapter.onPose(applyPose);
-      try {
-        await adapter.mount(state.street.host);
-      } catch (error) {
-        unsubscribe();
-        throw error;
-      }
+      await adapter.mount(state.street.host);
       if (mounting?.promise !== promise) {
-        // Unmounted (layer off, provider switched) while loading.
-        unsubscribe();
-        adapter.unmount();
+        // Unmounted (layer off, provider switched) while loading. A newer
+        // mount of this provider shares the adapter and its viewer: tearing
+        // it down here would leave that mount active with no viewer.
+        const reused =
+          active?.id === entry.def.id || mounting?.id === entry.def.id;
+        if (!reused) adapter.unmount();
         throw new Error('Street-level viewer was closed');
       }
+      // Listen only once current: an outdated mount releasing the same
+      // `applyPose` would otherwise unhook the mount that replaced it.
+      const unsubscribe = adapter.onPose(applyPose);
       active = { id: entry.def.id, adapter, unsubscribe };
       warmed.delete(entry.def.id);
       adapter.setRenderMode?.(state.street.renderMode);