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.

Art/blender/street/street_mesh.py

@@ -218,13 +218,31 @@ def main():
         z = dem_z(x, y)
         return (z if z is not None else 0.0) + 0.02
 
-    def south_z(r, off, x, y):
+    def sidewalk_z(r, off, inner, outer, x, y):
+        """Sidewalk surface: full kerb upstand at the road-side (inner) edge,
+        tapering to zero at the building-side (outer) edge so the walk surface
+        meets the terrain flush.
+
+        Before this taper the outer lip stood the full kerb height above the DEM,
+        which the walkability pre-check measured as a 15.8 cm step where a sample
+        crosses from Terrain_DEM onto Sidewalk_South (with a matching lip on the
+        north side). A 0.15 m lip over a 4.0 m / 5.3 m strip is a ~3-4 % ramp:
+        walkable and visually negligible, and it removes the ledge entirely.
+        """
         z = dem_z(x, y)
         base = z if z is not None else 0.0
-        return base + kerb_height(rows[r][0], args.kerb, args.ramp_half_width, args.ramp_blend)
+        h = kerb_height(rows[r][0], args.kerb, args.ramp_half_width, args.ramp_blend)
+        span = inner - outer
+        if abs(span) < 1e-9:
+            return base + h
+        t = min(1.0, max(0.0, (off - outer) / span))
+        return base + h * t
+
+    def south_z(r, off, x, y):
+        return sidewalk_z(r, off, SOUTH_GUTTER, SOUTH_SIDEWALK, x, y)
 
     def north_z(r, off, x, y):
-        return south_z(r, off, x, y)
+        return sidewalk_z(r, off, NORTH_GUTTER, NORTH_SIDEWALK, x, y)
 
     road = strip(rows, road_offsets, road_z, "Road_Hollywood", asphalt)
     sw_s = strip(rows, south_offsets, south_z, "Sidewalk_South", concrete)