fix(street-level): settle loading on pan, keep zoom 0, drop late poses

Fixes from gekh's re-review of 061d269, each with a regression test that
fails on the old code:

- A pan that drops a tile still loading now settles the loading count
  (the aborted request no longer owns its key, so its `finally` never
  did), and purges the previous zoom's tiles once nothing is pending.
  LOADING no longer sticks on the status pill and chip.
- Zoom 0 (above 15,000 km) is a zoom: `zoom == null` instead of `!zoom`.
- A pose that arrives after a photo was closed is dropped, in the
  MapillaryJS adapter and in the viewer host, so the marker and sequence
  selection do not come back.
- Ground casting passes the tile's signal to the terrain request, so a
  cancelled tile stops its height batch instead of holding the queue.
- The server's in-memory tile cache honours the 24 h TTL like the disk.
- The viewer's one-time resize runs for every photo, not just the first.

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

src/ui/streetLevelControls.test.mjs

@@ -500,4 +500,7 @@ test('the viewer is resized when it opens, not on every render (review #8)', ()
     assert.equal(layer.calls.resize, 1, 'once on open');
     for (let i = 0; i < 5; i++) layer.publish(uiState({ open: true }));
     assert.equal(layer.calls.resize, 1, 'not again while it stays open');
+    layer.publish(uiState({ open: false }));
+    layer.publish(uiState({ open: true }));
+    assert.equal(layer.calls.resize, 2, 'the next photo resizes again');
   }));