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/styles/street-level.css

@@ -118,7 +118,7 @@
   color: #ff8a80;
 }
 
-/* ── Filters and legend ──────────────────────────────────────────────────── */
+/* ── Filters ──────────────────────────────────────────────────── */
 .sl-filters {
   display: flex;
   align-items: center;
@@ -196,42 +196,6 @@
   color: var(--text-primary);
 }
 
-.sl-legend {
-  display: flex;
-  flex-wrap: wrap;
-  gap: 4px 12px;
-  margin: 0;
-  padding: 0;
-  list-style: none;
-}
-
-.sl-legend li {
-  display: inline-flex;
-  align-items: center;
-  gap: 6px;
-  font-family: var(--font-mono);
-  font-size: var(--sl-text);
-  letter-spacing: 0.06em;
-  color: var(--text-secondary);
-}
-
-.sl-legend[hidden] {
-  display: none;
-}
-
-.sl-legend-swatch {
-  width: 14px;
-  height: 3px;
-  border-radius: 2px;
-  display: inline-block;
-  background: var(--sl-green);
-}
-
-/* COLORS.selected in src/layers/streetLevel/policy.js */
-.sl-legend-swatch.is-selected {
-  background: #00d4ff;
-}
-
 /* ── Embedded viewer ─────────────────────────────────────────────────────── */
 .sl-viewer-wrap {
   position: relative;