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/street/street_mesh.py

@@ -68,8 +68,13 @@ def parse_args():
     ap.add_argument("--cross", type=float, default=0.75)
     ap.add_argument("--kerb", type=float, default=0.15)
     ap.add_argument("--ramp-half-width", type=float, default=2.5)
-    ap.add_argument("--ramp-blend", type=float, default=1.5)
-    return ap.parse_args(argv)
+    ap.add_argument("--ramp-blend", type=float, default=None,
+                    help="override the ramp run; default derives it from --max-ramp-grade")
+    ap.add_argument("--max-ramp-grade", type=float, default=0.08,
+                    help="maximum crossing-ramp slope (0.08 = 8%%)")
+    args = ap.parse_args(argv)
+    args.ramp_blend = ramp_blend(args.kerb, args.max_ramp_grade, args.ramp_blend)
+    return args
 
 
 def find_proj():
@@ -125,7 +130,13 @@ class Dem:
 
 
 def kerb_height(s, base, half_width, blend):
-    """Full kerb, except flushed near a crossing so people can step off."""
+    """Full kerb, except flushed near a crossing so people can step off.
+
+    `blend` is the horizontal run over which the kerb drops from `base` to the
+    0.02 m flush height. It is derived from a target maximum grade (see
+    `ramp_blend`) so the crossing ramps stay within a real ramp's slope instead
+    of the ~8.7 % the old fixed 1.5 m run produced.
+    """
     for crossing in CROSSINGS:
         d = abs(s - crossing)
         if d <= half_width:
@@ -136,6 +147,22 @@ def kerb_height(s, base, half_width, blend):
     return base
 
 
+def ramp_blend(kerb, max_grade, override=None):
+    """Horizontal run needed for the kerb ramp to stay at or under max_grade.
+
+    0.15 m kerb at an 8 % grade needs 1.63 m of run; the previous fixed 1.5 m
+    gave 8.7 %, and the walk pre-check measured the transition as a 10-13 cm
+    step over 1.5 m. When `override` is given it wins (kept for reproducing old
+    meshes), otherwise the run is computed from the live kerb height so it stays
+    correct if the kerb height assumption changes.
+    """
+    if override is not None:
+        return max(0.0, override)
+    if max_grade <= 0.0:
+        return 0.0
+    return max(0.0, (kerb - 0.02)) / max_grade
+
+
 def strip(rows, offsets, z_at, name, material):
     """Build a quad grid from per-row sampled vertices."""
     verts, faces = [], []