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  failure and retry, a superseded late answer, the cache and clearSelection.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/registry.test.mjs

@@ -9,9 +9,7 @@ const def = (overrides = {}) => ({
   requiresKeyId: 'mapillary',
   pickPrefix: 'mly:',
   colors: { coverage: '#05cb63' },
-  credit: { key: 'mapillary', html: 'Mapillary' },
-  capabilities: { coverage: 'tiles', sequences: true },
-  externalUrl: (id) => `https://example.test/${id}`,
+  credit: { html: 'Mapillary' },
   create: () => ({}),
   ...overrides,
 });
@@ -42,8 +40,6 @@ test('every required field is checked by name', () => {
     'pickPrefix',
     'colors',
     'credit',
-    'capabilities',
-    'externalUrl',
     'create',
   ])
     assert.throws(
@@ -52,10 +48,7 @@ test('every required field is checked by name', () => {
       key,
     );
   assert.throws(() => validateProviders([def({ create: 1 })]), /create/);
-  assert.throws(
-    () => validateProviders([def({ credit: { key: 'x' } })]),
-    /credit/,
-  );
+  assert.throws(() => validateProviders([def({ credit: {} })]), /credit/);
   assert.throws(
     () => validateProviders([def({ colors: { pano: '#f0f' } })]),
     /coverage colour/,