feat(site): slice the ground into 20 m segments - and the slice hypothesis dies with them

The site exporter now splits sidewalks and kerbs into ~20 m pieces
(SEGMENTED / SEGMENT_M in Art/blender/site/export_site_fbx.py), 56 slices in all, so each has a
bounding box that means something where it stands instead of reporting the highest point on a
279 m slope. They import and stage (63 site meshes in the demo map).

The hypothesis behind it is dead, and that is the useful part: with the invisible sidewalk
stopgap retired, the player now falls to Road_Hollywood at 366 - the slices do not block either.
So the 279 m aspect ratio was never the cause; a 20 m slice of the same mesh fails identically.
The sidewalk geometry itself is what does not collide, and the next place to look is its shape -
an open surface with no volume, inverted winding or zero-thickness triangles - not the import
flags and not the size.

The stopgap retirement stands: the player walks on the road, which is real geometry that does
collide, rather than on an invisible ledge.

MEMORY.md

@@ -519,3 +519,10 @@ Scope: facts and traps that cost time to establish. Planned work lives in [TODO.
 - **Likely cause: the auto-generated simple collision for a 279 m x 8.6 m x 3.8 m slab is degenerate.** A convex hull over that aspect ratio either collapses or ends up far from the surface; the complex side is one-sided and the mesh may be wound the wrong way. `bOneSidedGeometry` no longer exists on `BodySetup` in 5.8, so the flag cannot be flipped from Python.
 - **Working stopgap:** an invisible `279 x 8.6 x 0.08 m` blocking cube with its top at Z 380 in `Tools/ue_build_demo_map.py`. Verified: `top surface under player: Z=380 mesh=Cube`, `foot: actor Z 470, feet 382`.
 - **The real fix belongs in the site export:** split the sidewalk into segments of a few metres, or export explicit `UCX_` boxes, so the collision is a set of sane hulls rather than one degenerate one.
+
+## 2026-11-09 — Slicing the sidewalk did not fix its collision
+
+- **The 20 m slice hypothesis is dead.** The site export now splits sidewalks and kerbs into ~20 m pieces (`Art/blender/site/export_site_fbx.py`, `SEGMENTED`, `SEGMENT_M`), so each has an honest per-slice bounding box. The slices still do **not** block: with the invisible stopgap retired, a downward trace goes straight to `Road_Hollywood` at 366 and the capsule lands there.
+- **So the slab's 279 m aspect ratio was not the cause** of the missing collision — a 20 m slice of the same mesh fails identically. The mesh itself imports with collision that does not block, regardless of size.
+- **What does collide:** `Road_Hollywood` (366) and the engine cube stopgap. What does not: every `Sidewalk_*` and `Kerb_*` slice.
+- **Next place to look:** the sidewalk mesh's geometry - an open surface with no volume, inverted winding, or zero-thickness triangles - rather than the import flags or the size.