Set buildings with security alerts on fire and tape them off

After a GitHub snapshot loads, the viewer's code scanning and Dependabot
alerts are fetched and grouped by file. In the city, affected buildings
smoke, and those with a critical or high alert also burn: GPU particles
whose age derives from time and seed, so emitters upload once. Hazard
tape runs diagonally across every wall, yellow DEPENDABOT ALERT rising
and red CODE SCANNING ALERT falling, crossing as an X when a file has
both; the tape tiles a striped label cut to the wall's diagonal.

The minimap marks alert buildings, the coverage panel shows the alert
count (or why none could be loaded, e.g. signed out or no permission),
and a selected file lists its alerts with severity and GitHub links.

webtest/labels.test.mjs

@@ -80,3 +80,32 @@ test('folder labels keep the tail of long paths', () => {
   const long = folderLabel('web/ui/react-app/src/pages/graph/components', 'x');
   assert.ok(long.startsWith('…/') && long.endsWith('graph/components') && long.length <= 34 + 2, long);
 });
+
+import { alertsByPath, burns, tapeQuads } from '../web/city.js';
+
+test('alerts group by file with the worst severity', () => {
+  const byPath = alertsByPath({ available: true, codeScanning: { alerts: [{ path: 'a.go', severity: 'medium' }, { path: 'a.go', severity: 'critical' }] }, dependabot: { alerts: [{ path: 'go.mod', severity: 'low' }, { path: '', severity: 'high' }] } });
+  assert.equal(byPath.size, 2);
+  assert.equal(byPath.get('a.go').worst, 'critical');
+  assert.equal(byPath.get('a.go').codeScanning.length, 2);
+  assert.equal(byPath.get('go.mod').dependabot.length, 1);
+  assert.equal(alertsByPath({ available: false, reason: 'signin' }).size, 0);
+  assert.ok(burns('high') && burns('critical') && !burns('medium') && !burns(undefined));
+});
+
+test('hazard tape crosses each wall corner to corner, readable from outside', () => {
+  const b = { x: 0, y: 0, w: 20, h: 10, height: 40 };
+  for (const direction of [1, -1]) {
+    const quads = tapeQuads(b, 6, direction);
+    const south = quads.filter(q => q.c[1] > 10);
+    const length = south.reduce((sum, q) => sum + Math.hypot(...q.u) * 2, 0);
+    assert.ok(Math.abs(length - Math.hypot(20, 40)) < 1e-6, `covers the diagonal: ${length}`);
+    const [first, last] = [south[0], south.at(-1)];
+    assert.ok(direction > 0 ? first.c[2] < last.c[2] : first.c[2] > last.c[2], 'runs the right way');
+    assert.ok(south.every(q => q.c[2] >= 0 && q.c[2] <= 40 && q.c[0] >= 0 && q.c[0] <= 20));
+    assert.ok(south.every(q => readableFrom(q, [10, 60, 20])) && south.every(q => !readableFrom(q, [10, 5, 20])));
+    assert.ok(south.at(-1).fraction <= 1 && south.slice(0, -1).every(q => q.fraction === 1));
+    assert.equal(new Set(quads.map(q => `${Math.sign(q.c[0] - 10)},${Math.sign(q.c[1] - 5)}`)).size >= 4, true, 'all four walls');
+  }
+  assert.equal(tapeQuads({ x: 0, y: 0, w: 2, h: 2, height: 30 }, 6).length, 0, 'too narrow for tape');
+});