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  failure and retry, a superseded late answer, the cache and clearSelection.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/policy.js

@@ -1,5 +1,7 @@
 import { COLORS as SHARED_COLORS, PROVIDER_COLORS } from '../../policy.js';
 
+export { NEAREST_RADIUS_M } from '../../policy.js';
+
 /** Identity and tuning for the Mapillary street-level provider. */
 export const MAPILLARY_PROVIDER_ID = 'mapillary';
 export const MAPILLARY_NAME = 'Mapillary';
@@ -7,6 +9,14 @@ export const MAPILLARY_LABEL = 'MAPILLARY';
 export const MAPILLARY_KEY_ID = 'mapillary';
 export const MAPILLARY_GRAPH_HOST = 'https://graph.mapillary.com';
 
+/** Methods a source (./source.js or a stand-in) must have for the provider. */
+export const MAPILLARY_SOURCE_METHODS = Object.freeze([
+  'getStatus',
+  'getTile',
+  'getSequenceImages',
+  'nearestImages',
+]);
+
 /** Stable id prefixes for picked primitives; every one starts with `mly:`. */
 export const PICK_PREFIX = Object.freeze({
   root: 'mly:',
@@ -33,7 +43,7 @@ export const KEY_REJECTED_MESSAGE =
   'Mapillary rejected MAPILLARY_CLIENT_TOKEN — replace it in Provider Settings';
 /** Shown while Mapillary rate-limits tile requests (429). */
 export const RATE_LIMITED_MESSAGE =
-  'Mapillary is rate-limiting coverage requests — it resumes on its own';
+  'Coverage requests are being rate-limited — they resume on their own';
 /**
  * How far from the camera a street-zoom view may reach for coverage: a tilted
  * view sees to the horizon, but only the ground within this range is ranked
@@ -52,8 +62,7 @@ export const SEQUENCE_IMAGES_LIMIT = 2000;
 export const IMAGE_CONE_SIZE_PX = 26;
 export const IMAGE_CONE_MIN_SPACING_M = 3;
 
-/** Nearest-image search when the user asks to look at a place. */
-export const NEAREST_RADIUS_M = 50;
+/** Images asked for in a nearest-image search (radius: NEAREST_RADIUS_M). */
 export const NEAREST_LIMIT = 8;
 
 /**