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/streetLevelControls.test.mjs

@@ -223,7 +223,6 @@ function panelDom({ fullscreen = true, popover = false } = {}) {
     add(controls, 'button', { dataset: { slPano: pano } });
   add(controls, 'input', { id: 'sl-since' });
   add(controls, 'output', { id: 'sl-since-label' });
-  add(root, 'ul', { id: 'sl-legend' });
   add(root, 'div', { id: 'sl-coverage-meta' });
   return { document, root, wrap };
 }
@@ -475,11 +474,6 @@ test('a layer that is on with Mapillary switched off reads OFF, and the pill bri
       dom.root.querySelector('#sl-coverage-meta').textContent,
       /Mapillary is off in this view/,
     );
-    assert.equal(
-      dom.root.querySelector('#sl-legend').hidden,
-      true,
-      'no legend',
-    );
     status.click();
     await settle();
     assert.deepEqual(calls.setParams, [