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/layers/streetLevel/viewerHost.test.mjs

@@ -27,8 +27,7 @@ registerHooks({
     return { format: 'module', shortCircuit: true, source };
   },
 });
-const { createMapillaryViewer } =
-  await import('./providers/mapillary/viewer.js');
+const { createMapillaryViewer } = await import('./viewer.js');
 
 /**
  * A viewer host over `adapter`. `marker` records where the position marker