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/ui/templates/layer-panels.html

@@ -124,10 +124,6 @@
             <output id="sl-since-label" class="sl-since-label" for="sl-since">ANY DATE</output>
           </div>
         </fieldset>
-        <ul id="sl-legend" class="sl-legend" aria-label="Coverage colours">
-          <li><i class="sl-legend-swatch"></i><span>Mapillary</span></li>
-          <li><i class="sl-legend-swatch is-selected"></i><span>Selected</span></li>
-        </ul>
         <div id="sl-coverage-meta" class="sl-meta"></div>
       </div>
     </div>