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]>

src/ui/streetLevelControls.test.mjs

@@ -435,25 +435,6 @@ test('under KEY REQUIRED the filters are gated and the error line says how to ad
     controls.destroy();
   }));
 
-test('the legend rebuilds when a swatch changes, even at the same count', () =>
-  withDom(async (dom) => {
-    const { layer, controls } = stubPanel(dom, uiState());
-    const legend = dom.root.querySelector('#sl-legend');
-    const swatch = () => legend.children[0].children[0].style.background;
-    assert.equal(swatch(), '#05cb63');
-    layer.publish(
-      uiState({
-        legend: [
-          { key: 'mapillary', label: 'Mapillary', color: '#a66bff' },
-          { key: 'selected', label: 'Selected', color: '#00d4ff' },
-        ],
-      }),
-    );
-    assert.equal(legend.childElementCount, 2);
-    assert.equal(swatch(), '#a66bff');
-    controls.destroy();
-  }));
-
 test('the viewer is resized once when it opens, not on every render', () =>
   withDom(async (dom) => {
     const { layer, controls } = stubPanel(dom, uiState());