# 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.

## Requirements

- 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.

## Acceptance Criteria

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

## Verification Record

- Native geometry and Workbench tests passed.
- Nantaizi native compile/check passed with `ok=true`.
- Native Blender/Cesium/preview verification was completed manually in a
  working Blender environment.

## Notes

- 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`.