fix(street): taper sidewalk upstand to flush at the building edge

The sidewalk stood the full 0.15 m kerb height above the DEM at *both* edges, so
the outer (building-side) lip was a 15.8 cm ledge where the walk route crossed
from Terrain_DEM onto Sidewalk_South. sidewalk_z now tapers the upstand from full
at the road-side edge to zero at the building-side edge (a ~3-4 % ramp, walkable
and visually negligible).

Pre-check: south max step 20.5 -> 13.4 cm, north 15.1 -> 7.2 cm (the residual
13.4 cm is the crossing-ramp gradient, tracked separately). Street and site
rebuilt, site re-imported (7/7 meshes, collision flags intact), DemoMap rebuilt;
packaged -qaselftest still PASS. README step table updated.

TODO.md

@@ -15,7 +15,7 @@ Suggested blender skills: **collision-proxy** (UCX/UHX naming, convex hulls, sim
 
 ### Patch tasks - active - 01a1b26b-3c94-707f-9665-2f6b4a79b564 (branch `collision-detail-0.1.1`)
 
-- [A] **Sidewalk outer edge against terrain.** The pre-check measured a **15.8 cm** step where a walk sample crosses from `Terrain_DEM` onto `Sidewalk_South` (`Art/blender/README.md`, s≈207 m). Mostly hidden by buildings; visible wherever the frontage is missing. Fix by extending the sidewalk mesh to meet the terrain, or by lowering the outer lip.
+- [x] **Sidewalk outer edge against terrain.** **Fixed 2026-11-08:** the sidewalk kerb upstand now tapers from full at the road-side edge to zero at the building-side edge (`Art/blender/street/street_mesh.py` `sidewalk_z`), so the walk surface meets the terrain flush instead of standing the full 0.15 m kerb above the DEM. Pre-check after the change: south max step 20.5 → **13.4 cm**, north 15.1 → **7.2 cm** (the remaining 13.4 cm is the crossing-ramp item below). Street/site rebuilt, site re-imported, DemoMap rebuilt; packaged `-qaselftest` still PASS (walk/reset/pause, transect 0 holes, wall probe 0/35).
 - [ ] **Crossing ramp gradient.** The kerb-flush transitions measure **10–13 cm over 1.5 m ≈ 10 %**, steeper than the 8 % a real ramp uses (`Geo/tools/street_section.py` output, s≈33, 39, 126 m). Grade the three ramp openings properly.
 - [x] **Collision authoring pass.** Terrain and site meshes use complex-as-simple / `BlockAll` (forced by `UGigLineAssetTools`, verified 2026-11-08 with 84-sample transect). Remaining: simple convex hulls per building (currently using default collision). Record the final settings in `Docs/build.md`.
 - [ ] **Kerb height is an assumption.** 0.15 m modelled, 0.13 m as built, and the 3DEP DEM cannot resolve a kerb at all. Check against the photographs and record the final figure.