Route dependents to the selected file with pulsing magenta dashes

Files that import the selected file now get street routes too, from
the same single search (paths cost the same both ways), reversed so
they flow toward the selection. They draw as magenta dashes marching
inward with a pulse travelling along them, dashed on the minimap too.
Ribbon geometry moves into city.js and every ribbon keeps to the right
of its direction of travel, so an import and a dependent sharing a
street run in separate lanes instead of overlapping (tested).

webtest/nav.test.mjs

@@ -112,3 +112,18 @@ test('posters are pasted on the city side of the walls, tilted and readable from
   }
   assert.ok(new Set(posters.map(p => p.u[2].toFixed(2))).size > 10, 'arbitrary angles');
 });
+
+import { ribbonVertices, heading } from '../web/city.js';
+
+test('route ribbons keep right, so opposite directions on one street use separate lanes', () => {
+  const east = ribbonVertices([{ points: [[0, 0], [100, 0]], kind: 0 }]);
+  const west = ribbonVertices([{ points: [[100, 0], [0, 0]], kind: 2 }]);
+  const ys = vertices => { const out = []; for (let i = 1; i < vertices.length; i += 6) out.push(vertices[i]); return out; };
+  // Heading east (+x) the walker's right is heading(yaw).right with forward (1, 0): yaw = -PI/2.
+  const right = heading(-Math.PI / 2).right;
+  assert.ok(Math.abs(right[0]) < 1e-9 && right[1] > 0, 'right of east is +y here');
+  assert.ok(ys(east).every(y => y > 0), `eastbound ribbon on +y: ${ys(east)}`);
+  assert.ok(ys(west).every(y => y < 0), `westbound ribbon on -y: ${ys(west)}`);
+  const along = []; for (let i = 4; i < east.length; i += 6) along.push(east[i]);
+  assert.ok(Math.max(...along) > 99 && Math.min(...along) < 0 && east[5] === 0 && west[5] === 2);
+});