refactor(python): extract geometry/primitives.py from road.py

[nigel-hall-codes]

Migration plan: sdlc/#417/audit.md (root: #417). PR 6. Move the 8 shared underscore helpers (_sample_elevation, _clip_polyline_to_rect, _densify_polyline, ...) out of road.py with public names; update the 5 import sites (building, intersection, parking, sidewalk, stamping). Verify: V-BAKE byte-identical rebake; pytest green. Addresses P1.