feat: smooth native junction boundaries

.trellis/tasks/08-17-native-rounded-junctions/check.jsonl

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+{"file":".trellis/spec/pipeline/cli-and-stages.md","reason":"Native geometry correctness and output contracts."}
+{"file":".trellis/spec/blender/testing.md","reason":"Blender validation requirements."}

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

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+# Design
+
+Each approach contributes its two carriageway-edge points at the common
+cutback distance. Points are ordered around the junction node. For each pair
+from adjacent approaches, the compiler samples a deterministic quadratic
+Bezier whose control point follows the pedestrian-side curb arc toward the
+junction. Rounded plans use a larger cutback than the legacy straight envelope
+so this visible curb shape still contains all turning connectors. Approach road
+surfaces terminate at the same cutback, so they cannot cover the junction
+outline in 3D output.
+
+The curve is accepted only when the support intersection is finite, the pair
+belongs to different approaches, and the resulting ring remains valid and
+contains all published connector coordinates. Otherwise the original straight
+chord remains for that corner and the plan reports a mixed/fallback boundary.
+
+Lane connectors remain a separate vehicle-path layer. This task changes only
+the road/intersection outline and sidewalk-corner shape.

.trellis/tasks/08-17-native-rounded-junctions/implement.jsonl

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+{"file":".trellis/spec/pipeline/cli-and-stages.md","reason":"Native geometry and build-stage contracts."}
+{"file":".trellis/spec/guides/artifact-parity-guide.md","reason":"Intentional geometry output change validation."}

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

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+# Implementation
+
+1. Build a rounded junction boundary from ordered approach-edge records with
+   tangent support-line intersections and deterministic curve samples.
+2. Expose boundary mode/provenance and preserve containment fallback.
+3. Extend focused native-road tests for curved ordinary intersections.
+4. Validate Nantaizi compile/check, workbench tests, and native 3D build.

.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`.

.trellis/tasks/08-17-native-rounded-junctions/task.json

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+{
+  "id": "native-rounded-junctions",
+  "name": "native-rounded-junctions",
+  "title": "Rounded native road junctions",
+  "description": "",
+  "status": "in_progress",
+  "dev_type": null,
+  "scope": null,
+  "package": null,
+  "priority": "P2",
+  "creator": "dingkang",
+  "assignee": "dingkang",
+  "createdAt": "2026-08-17",
+  "completedAt": null,
+  "branch": null,
+  "base_branch": "feature/native-road-compiler",
+  "worktree_path": null,
+  "commit": null,
+  "pr_url": null,
+  "subtasks": [],
+  "children": [],
+  "parent": "08-13-native-road-compiler",
+  "relatedFiles": [],
+  "notes": "",
+  "meta": {}
+}