refactor(street-level): drop hover, legend and byte-budget cache; flatten the provider directory

- No pointer cursor over coverage lines (no other layer has one); a click
  test replaces the hover tests.
- No legend: one source, one colour.
- The on-globe credit lives in index.js; credits.js is gone.
- Billboard glyphs are SVG data URLs with the same geometry, so tests need
  no canvas stub.
- The tile cache keeps the 120 most recently used tiles, same 24 h TTL,
  instead of a byte budget; listTileLayers was test-only.
- providers/mapillary/ is flattened into the layer and its policy merged
  into one policy.js; the source's method list is a literal in the catalog.
- One fakeCesiumViewer for the layer tests.
- The fixture gate keeps the steps the unit tests do not cover: coverage and
  credit, a rejected key, line -> cone -> photo, close, a keyless page.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/app/constructCatalog.js

@@ -3,7 +3,6 @@ import { createWeatherLayer } from '../layers/weather/index.js';
 import { createCyclonesLayer } from '../layers/cyclones/index.js';
 import { createWindLayer } from '../layers/wind/index.js';
 import { createLayerCatalog } from './catalog.js';
-import { MAPILLARY_SOURCE_METHODS } from '../layers/streetLevel/providers/mapillary/policy.js';
 import { LAYER_STATE_REGISTRY } from '../data/layerState.js';
 import { createMilitaryRegistry } from '../layers/aircraft/classification.js';
 import { createApplicationFlights } from './layers/flights.js';
@@ -57,7 +56,7 @@ const SOURCE_METHODS = Object.freeze({
   earthquakes: ['getSnapshot'],
   'fire-perimeters': ['getSnapshot'],
   cables: ['fetch'],
-  mapillary: MAPILLARY_SOURCE_METHODS,
+  mapillary: ['getStatus', 'getTile', 'getSequenceImages'],
 });
 
 /**