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]>

CHANGELOG.md

@@ -73,7 +73,7 @@
 
 - Street Level: Mapillary street-level imagery (free client token, CC BY-SA
   4.0). A right-rail panel holds an ON/OFF pill, 360°/flat and captured-since
-  filters, a legend and an embedded viewer with EXPAND and FIT/FILL. Coverage
+  filters and an embedded viewer with EXPAND and FIT/FILL. Coverage
   lines (below 60 km) and image cones drape on terrain and Google 3D tiles;
   opening a photo frames it with one camera flight. Share links carry the
   switch, the filters and the panel's collapsed state. A server proxy adds the