fix(street-level): sequence loads, playback and nearest-photo edge cases

From gekh's review of #768, each bug with a test that fails before:

- P2: a failed sequence load clears the previous sequence's cones, resets
  the selection and can be retried; a second click while loading starts no
  second lookup. sequences.js had no tests: sequences.test.mjs now covers
  failure and retry, a superseded late answer, the cache and clearSelection.
- P2: closing a photo stops MapillaryJS sequence playback, which kept the
  hidden viewer stepping and downloading images.
- openNearest without the panel reports it instead of loading forever, and
  stops if the layer (or its provider) went off during the lookup; the
  mesh sampler's misses are bounded and forgotten with its samples; the
  sequence count is the union of ids across tiles; a provider withdraws
  only its own error, so a sequence error cannot mask KEY REJECTED; hover
  stays on (throttled) under an orbiting camera and re-picks when it
  stops; "since N days" follows the clock in a long-open tab.
- A prewarm waits for one in flight and then builds if it gave up.
- New coverage for the layer core (credits, KEY REJECTED vs REQUIRED,
  switching a provider off), the refresh wiring of the tilted-view fix and
  the draped-to-cast swap completion.
- Cleanup: loading is derived from pending; freshStreet(); tile ranking
  computes distances once and shares one path for the date line; shared
  metresBetween / whenIdle / redrawTile; one NEAREST_RADIUS_M and one
  source method list; unused contract fields, exports, getImage and the
  decoder's image branch removed; PROVIDER_COLORS lists only Mapillary.

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

src/layers/streetLevel/providers/mapillary/source.test.mjs

@@ -83,7 +83,7 @@ test('graph calls without a token fail fast as keyRequired without a request', a
   const { calls, fetchImpl } = fakeFetch(() => jsonResponse({}));
   const source = createMapillarySource({ token: '', fetchImpl });
   await assert.rejects(
-    () => source.getImage('123'),
+    () => source.getSequenceImages('123'),
     (error) =>
       error instanceof MapillarySourceError && error.keyRequired === true,
   );
@@ -96,7 +96,7 @@ test('graph API errors carry the upstream message and status', async () => {
   );
   const source = createMapillarySource({ token: 'MLY|1|abc', fetchImpl });
   await assert.rejects(
-    () => source.getImage('999'),
+    () => source.getSequenceImages('999'),
     (error) =>
       error instanceof MapillarySourceError &&
       error.status === 400 &&