Rounded native road junctions

Goal

Replace the octagonal native junction outline with smooth, tangentially joined road-edge corners for ordinary Nantaizi T and cross junctions.

Junction surface boundaries must connect adjacent approach carriageway edges with a smooth outward curve rather than a straight octagonal chord.
Sidewalk corner surfaces must use the same rounded boundary concept so road and pedestrian geometry do not disagree visually.
Preserve a deterministic straight-edge fallback and an explicit diagnostic when a corner cannot be safely constructed.
Do not change lane connector semantics or derive geometry from osm2streets.

Ordinary cross/T fixtures generate rounded junction polygons with more than the prior eight straight boundary vertices and boundary_mode records the chosen style.
Connector containment remains valid and degenerate geometry falls back without publishing self-intersecting polygons.
Nantaizi compile/check and native Blender/Cesium/preview succeed.

Keep prd.md focused on requirements, constraints, and acceptance criteria.
Lightweight tasks can remain PRD-only.
For complex tasks, add design.md for technical design and implement.md for execution planning before task.py start.