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/marker.test.mjs

@@ -3,13 +3,6 @@ import test from 'node:test';
 import * as Cesium from 'cesium';
 import { createMarker } from './marker.js';
 
-// The marker glyph draws on a canvas; Node has none, so drawing is a no-op.
-const noop = () => {};
-const context2d = new Proxy({}, { get: () => noop, set: () => true });
-globalThis.document ??= {
-  createElement: () => ({ getContext: () => context2d }),
-};
-
 /** A viewer whose camera is 20,000 km over `view`, and its pre-render listeners. */
 function setup() {
   const view = { lon: -121.49, lat: 38.58, height: 20_000_000 };