fix(street-level): prepare ground heights for the mesh densifying too

castLine densifies at 0.0002° when mesh heights refine it, but prepareLines
only fetched grid corners for the 0.0005° points. The finer points could
land in cells nobody fetched, so the cast came back null and the whole line
stayed draped over roofs and trees on Google 3D. prepareLines now prepares
both sets (corners are deduplicated, so the extra cost is small).

A seeded test of 300 walks with 50-300 m segments: 130 failed before, 0 now.
Found in the IC8 review of #768 (P1).

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

src/layers/streetLevel/groundCast.js

@@ -206,10 +206,18 @@ export function createGroundCaster({
     return run;
   }
 
-  /** `prepare` for lines, including the points densifying adds. */
+  /**
+   * `prepare` for lines, including the points densifying adds. `castLine`
+   * densifies finer when mesh heights refine it, and those points can fall in
+   * grid cells the coarse points skip, so both sets are prepared: otherwise
+   * the cast comes back null and the line stays draped over roofs and trees.
+   */
   function prepareLines(lines, options) {
     return prepare(
-      lines.flatMap((coords) => densifyLine(coords)),
+      lines.flatMap((coords) => [
+        ...densifyLine(coords),
+        ...densifyLine(coords, MESH_DENSIFY_DEG),
+      ]),
       options,
     );
   }

src/layers/streetLevel/groundCast.test.mjs

@@ -254,3 +254,31 @@ test('cancelling a prepare cancels its terrain request', async () => {
   assert.equal(seen.length, 1);
   assert.equal(seen[0], controller.signal, 'the tile signal reaches the proxy');
 });
+
+test('prepared lines always cast with the mesh, however their segments cross the grid (review IC8 P1)', async () => {
+  // Seeded walks with 50–300 m segments: the finer mesh densifying used to
+  // land in grid cells the coarse points skipped, so most of them came back
+  // null and stayed draped over roofs.
+  let seed = 7;
+  const random = () => {
+    seed = (seed * 16807) % 2147483647;
+    return seed / 2147483647;
+  };
+  let failures = 0;
+  for (let n = 0; n < 300; n++) {
+    const caster = createGroundCaster({ terrain: fakeTerrain() });
+    const line = [[10 + random() * 0.01, 50 + random() * 0.01]];
+    for (let k = 0; k < 10; k++) {
+      const [lon, lat] = line[line.length - 1];
+      const length = (50 + random() * 250) / 111_000;
+      const angle = random() * 2 * Math.PI;
+      line.push([
+        lon + length * Math.cos(angle),
+        lat + length * Math.sin(angle),
+      ]);
+    }
+    assert.equal(await caster.prepareLines([line]), true);
+    if (caster.castLine(line, { meshAt: () => undefined }) === null) failures++;
+  }
+  assert.equal(failures, 0);
+});