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  failure and retry, a superseded late answer, the cache and clearSelection.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

@@ -7,12 +7,11 @@ export const POSITION_PICK_ID = 'sl:pos';
 /**
  * One colour per imagery source, used everywhere that source appears: its
  * chip, coverage lines, overview points, image cones and legend swatch.
- * Registered providers take their entry from here; a new source adds one.
+ * Registered providers take their entry from here; a new source adds its
+ * colour together with its provider.
  */
 export const PROVIDER_COLORS = Object.freeze({
   mapillary: '#05cb63',
-  panoramax: '#a66bff',
-  'google-street-view': '#4285f4',
 });
 
 /** Colours every provider shares: the selection highlight and the marker. */
@@ -30,7 +29,10 @@ export const MAX_SINCE_DAYS = 36_500;
 /** Filter every provider starts with: all imagery, any date. */
 export const FILTER_DEFAULT = Object.freeze({ pano: 'all', sinceDays: 0 });
 
-/** Nearest-image search when the user asks to look at a place. */
+/**
+ * Nearest-image search when the user asks to look at a place, in metres
+ * (Mapillary's graph API answers within at most 50 m).
+ */
 export const NEAREST_RADIUS_M = 50;
 
 /**