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.

Art/blender/README.md

@@ -108,17 +108,30 @@ A downward raycast on a 1 m grid along both sidewalks and the designated crossin
 
 | Route             | Samples | Holes | Z range       | Worst step |
 | ----------------- | ------: | ----: | ------------- | ---------: |
-| South sidewalk    |     279 |     0 | −0.47 … +3.06 |    13.4 cm |
-| North sidewalk    |     279 |     0 | +0.07 … +3.76 |     7.2 cm |
-| Crossing at 130 m |      26 |     0 | +1.14 … +1.80 |     9.0 cm |
+| South sidewalk    |     279 |     0 | −0.48 … +3.06 |    13.4 cm (s=198) |
+| North sidewalk    |     279 |     0 | +0.06 … +3.76 |     6.9 cm (s=33) |
+| Crossing at 130 m |      26 |     0 | +1.14 … +1.80 |     9.0 cm (off −8.5) |
+
+At `--sample 0.25` the same routes measure 11.6 cm / 8.0 cm / 3.4 cm. The
+pre-check now prints the grade and the **location** of the worst step, and its
+coverage no longer shrinks with `--sample`.
 
 No holes anywhere, so nothing drops the player on either side. The two flagged steps:
 
 - ~~15.8 cm at s≈207, the sidewalk's outer edge~~ — **fixed in `0.1.1`**: the
   sidewalk upstand now tapers to zero at the building side, so the outer edge is
-  flush with the terrain. The remaining 13.4 cm max step on the south route is the
-  crossing-ramp gradient below, not the outer edge.
-- **10–13 cm steps at s≈33, 39 and 126**, inside the sidewalk mesh: the kerb-flush transition at the crossings. That is a ~10 % ramp over 1.5 m, steeper than a real crossing ramp (8 % max). Worth grading properly when the crossings are detailed.
+  flush with the terrain (north route 15.1 → 6.9 cm).
+- ~~10–13 cm steps at the crossings, ~10 % ramp over 1.5 m~~ — **fixed in `0.1.1`**:
+  the ramp run is now derived from a target grade (`--max-ramp-grade`, default 8 %),
+  giving 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 (~1.4 % of the route) at the kerb line.
+
+**Remaining finding (not a crossing, not the outer edge):** the south route still
+measures a **13.4 cm step at s≈198 m** (11.6 cm at 0.25 m sampling) and the north
+route 8.0 cm at s≈251 m. Neither is at a crossing and neither is the outer lip, so
+this is a local discontinuity in the sidewalk strip — most likely the DEM sampling
+or a centreline vertex near the block ends. Tracked under the `0.1.1` "audit any
+collision gaps" item rather than left undocumented.
 
 This is a pre-check, not the `0.1.0` release check: the release check is traversal in the engine.