feat(pipeline): shared centerline vertical-profile smoother for roads

[nigel-hall-codes]

Keystone of #230 (road smoothness). See sdlc/#230/plan.md, prior-art.md, analysis.md on branch sdlc/#230-idea.

Goal

Add a shared centerline vertical-profile smoother so roads stop reproducing high-frequency terrain noise, without regressing #219 (steep-grade conform).

What

• Add _smooth_centerline_profile() in python/sfmap/geometry/road.py.
• Apply it identically in:
  • Mesh pass — road.py::_build_single_road, after _anchor_centerline.
  • Stamp pass — stamping.py::stamp_roads, on sampled_y before _iter_road_footprint.
• Filter interior Y only; keep anchored endpoints and all XZ exact.
• Use a curvature / second-derivative limiter along arc length (preferred over a moving average so genuine straight grades pass untouched). Parameter = arc-length window in metres.

Critical invariant

Stamp grade and mesh grade must remain bit-identical for the same input — otherwise the road tears a seam (the exact failure #219 fixed).

Acceptance

• No seam/facet regression on steep grades (Lombard).
• High-frequency wiggle on flat input is attenuated.
• Ships the first python/tests/: endpoint + XZ preservation, stamp==mesh profile equality, straight-ramp passthrough, synthetic-noise attenuation.