fix(street): grade crossing ramps to <=8%; report step location in the pre-check

- kerb_height ramp run now derives from --max-ramp-grade (default 8%), giving
  1.63 m instead of the fixed 1.5 m (8.7%). Crossing route measures 3.4 cm at
  0.25 m sampling.
- site_assembly pre-check: coverage is independent of --sample (it used to shrink
  the route), and it now prints the grade and the s/offset location of the worst
  step so a step can be located instead of guessed.
- Documents the remaining south s=198 / north s=251 strip discontinuity as a
  finding on the 'audit collision gaps' item rather than claiming it fixed.
- Street + site rebuilt, site re-imported (7/7), DemoMap rebuilt; packaged
  -qaselftest PASS.

TODO.md

@@ -16,10 +16,10 @@ 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`)
 
 - [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] **Crossing ramp gradient.** **Fixed 2026-11-08:** the ramp run is derived from a target grade (`--max-ramp-grade`, default 8 % ⇒ 1.63 m) instead of the fixed 1.5 m (8.7 %). Measured at 0.25 m sampling the crossing now steps 3.4 cm at the kerb line. Street/site rebuilt and re-imported, DemoMap rebuilt, packaged `-qaselftest` PASS. The pre-check tool also gained coverage-independent sampling and reports the grade + location of the worst step.
 - [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.
-- [ ] Audit and fix any collision gaps or fall-through geometry found in `0.1.0` in the engine.
+- [ ] Audit and fix any collision gaps or fall-through geometry found in `0.1.0` in the engine. **Known finding to chase:** the walk pre-check reports a 13.4 cm step at **s≈198 m** on the south route and 8.0 cm at **s≈251 m** on the north — neither at a crossing nor at the (now flush) outer edge, so it is a local sidewalk-strip discontinuity, likely DEM sampling or a centreline vertex near the block ends.
 - [ ] Regression-test traversal on both sides of the demo.
 - [ ] Verify no new packaging defects.
 - [ ] Tag `v0.1.1` and write `CHANGELOG.md` entry; then remove this section.