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]>

server/providers/mapillary/trim.js

@@ -41,16 +41,3 @@ export function stripTileLayers(bytes, dropNames) {
   }, null);
   return dropped ? Buffer.from(writer.finish()) : bytes;
 }
-
-/** Names of the layers inside a vector tile, in order. */
-export function listTileLayers(bytes) {
-  const names = [];
-  if (!bytes?.length) return names;
-  new PbfReader(bytes).readFields((tag, _result, pbf) => {
-    if (tag !== TILE_LAYER_FIELD) return;
-    const end = pbf.readVarint() + pbf.pos;
-    names.push(layerName(bytes.subarray(pbf.pos, end)));
-    pbf.pos = end;
-  }, null);
-  return names;
-}