refactor(street-level): trim QA, tests, styles and docs

The browser gate is one pass over the remaining flows: rail strip and
expand, coverage with fixtures, the ON/OFF pill, 360° and SINCE filters,
KEY REJECTED, nearest photo with caption, × close, phone fit, keyless
KEY REQUIRED on a second page and no page errors. Panel controls are
pressed through the DOM; pick-point search, line/cone clicks, the
framing-cancel, reload/share and status-route probes, the second
viewport and pointer-settling retries are gone (unit tests cover them).

Tests that pinned constants, copy, CSS or source text, or repeated
another test, are removed or merged. CSS loses an unused variable and
duplicate rules. Docs describe the slimmed feature; unused exports and
the layer's dead setCoverageFilter/refreshCoverage surface are removed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

scripts/fixtures/street-level/mapillaryGraph.mjs

@@ -21,7 +21,7 @@ export const PHOTO_LINE = Object.freeze({
   north: 38.591,
 });
 /** Metres between consecutive photos on the line (≥ the 3 m cone thinning). */
-export const PHOTO_SPACING_M = 30;
+const PHOTO_SPACING_M = 30;
 /** Fixture thumbnail host (answered, never reached). */
 export const THUMB_HOST = 'qa-fixture.mapillary.com';
 
@@ -117,7 +117,7 @@ const json = (status, body) => ({
 });
 
 let photoBytes = null;
-export function photoJpeg() {
+function photoJpeg() {
   photoBytes ??= readFileSync(new URL('./street-640x320.jpg', import.meta.url));
   return photoBytes;
 }