feat: smooth native junction boundaries

.trellis/tasks/08-17-native-rounded-junctions/prd.md

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+# 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
+
+- [ ] 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.
+
+## 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`.