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/site/site_assembly.py

@@ -165,10 +165,16 @@ def main():
     def lat_lon(s, offset):
         return oc.point_at(line, _lengths, s, offset, lat0, lon0)
 
+    # Sample spacing is independent of coverage: walk the whole block at
+    # `--sample` metres, so a finer spacing measures the gradient instead of
+    # accidentally shortening the route (previously range(int(block_len)) with a
+    # 0.25 m spacing only covered a quarter of the block).
+    along = int(block_len / args.sample)
+    across = int(26.0 / args.sample)
     route_specs = {
-        "south sidewalk": [(s, -12.5) for s in [i * args.sample for i in range(int(block_len))]],
-        "north sidewalk": [(s, 13.0) for s in [i * args.sample for i in range(int(block_len))]],
-        "crossing at 130 m": [(130.0, o) for o in [i * args.sample - 12.5 for i in range(26)]],
+        "south sidewalk": [(i * args.sample, -12.5) for i in range(along)],
+        "north sidewalk": [(i * args.sample, 13.0) for i in range(along)],
+        "crossing at 130 m": [(130.0, i * args.sample - 12.5) for i in range(across)],
     }
     flat = [(spec, lat_lon(s, off)) for spec, samples in route_specs.items() for s, off in samples]
     local = project_to_local([ll for _spec, ll in flat], find_proj())
@@ -185,12 +191,20 @@ def main():
                 heights.append(None)
             else:
                 heights.append(z)
-        deltas = [abs(b - a) for a, b in zip(heights, heights[1:])
+        deltas = [(i, abs(b - a)) for i, (a, b) in enumerate(zip(heights, heights[1:]))
                   if a is not None and b is not None]
-        max_step = max(deltas) if deltas else float("nan")
+        worst_i, max_step = (max(deltas, key=lambda t: t[1]) if deltas else (0, float("nan")))
         known = [h for h in heights if h is not None]
+        grade = (max_step / args.sample) * 100 if deltas else float("nan")
+        # Position of the worst step, as arc length / cross offset, so the cause
+        # can be located instead of guessed.
+        if label == "crossing at 130 m":
+            where = f"s=130 off={(worst_i * args.sample - 12.5):+.2f} m"
+        else:
+            where = f"s={worst_i * args.sample:.1f} m"
         print(f"  {label:18} samples {len(heights):3}  holes {holes}  "
-              f"Z {min(known):+.2f}..{max(known):+.2f}  max step {max_step*100:.1f} cm")
+              f"Z {min(known):+.2f}..{max(known):+.2f}  max step {max_step*100:.1f} cm "
+              f"({grade:.1f} % grade) at {where}")
 
     out = os.path.abspath(args.out)
     os.makedirs(os.path.dirname(out), exist_ok=True)