test(profile): the road is a flat slab at 366 while the sidewalks slope

Added a -profile probe: one downward trace every 20 m along the street at the player's Y, naming
the surface each time. It settles the question the bounding boxes could not:

  x=-14336..-2336  z=366  Road_Hollywood   (flat)
  x=-336           z=302  Road_Hollywood
  x=1664           z=5    Terrain_DEM

The road is FLAT at 366 cm across the demo block, while the sidewalk slices follow the DEM and
descend 380 -> 104 cm. So the ~2 m gap is not an import artefact and not a collision problem: the
road itself does not follow the elevation the sidewalks follow.

That also explains why every sidewalk fix failed - the surface the player stands on is this flat
road, and the sloped sidewalk is metres below it.

Next: how rows is built in Art/blender/street/street_mesh.py. The road and the sidewalk read the same
dem_z(x, y), so a flat road means the sampler returns the same value for every row.

MEMORY.md

@@ -546,3 +546,17 @@ Scope: facts and traps that cost time to establish. Planned work lives in [TODO.
 - Checked for two `Demo_Road_Hollywood` actors after several import rounds: **none**. One actor, at the origin, identity scale, mesh Z spanning -44..366 cm.
 - So the 366 cm the trace reports at the player's XY is the road mesh's own local height there, not a stale duplicate and not a far-side hit. The sidewalk slice at the same X tops at 169 cm.
 - In the source blend at the same street position both surfaces sit around 1.4-1.6 m and agree within a kerb height, so the ~2 m gap appears **after** the mesh leaves Blender - which points at the import path (unit handling, or the Y mirror noted in `export_site_fbx.py`) rather than the strip generation.
+
+## 2026-11-09 — The road is FLAT at 366 while the sidewalks slope: that is the gap
+
+- Ground profile along Hollywood Blvd, traced every 20 m at the player's Y (`-probes -profile`):
+
+```
+x=-14336 z=366 Road_Hollywood     x=-2336 z=366 Road_Hollywood
+x=-12336 z=366 Road_Hollywood     x= -336 z=302 Road_Hollywood
+x=-10336 z=366 Road_Hollywood     x= 1664 z=  5 Terrain_DEM
+```
+
+- **The road is a flat slab at 366 cm** across the whole demo block, while the sidewalk slices follow the DEM and descend 380 -> 104 cm. The gap is therefore in the street generator: the road is not sampling the elevation per row the way the sidewalks do, or it was written with a single Z.
+- That also explains why every sidewalk fix failed - the street the player actually stands on is the flat road, and the sloped sidewalk is metres below it.
+- **Next:** inspect how `rows` is built in `Art/blender/street/street_mesh.py` - the road's `road_z` reads `dem_z(x, y)` from the same rows as the sidewalk, so a flat result means `dem_z` is returning the same value for every row (origin offset or a lookup that fails and falls back), and the fix is in that sampler.