feat: smooth native junction boundaries

.trellis/spec/pipeline/cli-and-stages.md

@@ -414,6 +414,49 @@ GeoJSON，native Blender 构建通过 `catalog.NATIVE_ROAD_LAYERS` 消费它们
 正确：道路方向箭头进入 `direction_arrows.geojson`；只有 OSM 明确标注的动作进入
 `turn_arrows.geojson`。工作台用两个开关呈现，Blender 复用同一现有箭头材质。
 
+## Native 普通路口圆角
+
+### 1. 范围与触发条件
+
+`compile-native-roads.js` 为普通 T / 十字路口生成 `intersection_surface.geojson`
+和 `sidewalk_surface.geojson` 的路口边界。路口道路面必须在同一 cutback 处结束，不能
+用未裁剪的道路矩形覆盖圆角边界。
+
+### 2. 几何契约
+
+- 相邻道路边缘使用两条支持切线的交点作为二次曲线控制点；采样段数由
+  `JUNCTION_CURVE_SEGMENTS` 统一控制。
+- 机动车路口边界、人行道内侧路缘和人行道外侧边界都必须使用同一切线圆角规则；外侧
+  不能只对内侧采样点做线性偏移，避免内外曲率不一致。
+- `boundary_mode` 使用 `rounded-approach-envelope`，无法安全构造的角保持确定性直线
+  回退，并写入 `junction-rounded-corner-fallback` warning。
+- 已发布的 connector 必须包含在最终边界内，边界退化或 connector 越界时才允许使用
+  `connector-convex-fallback`。
+
+### 3. 校验与错误矩阵
+
+| 条件 | 结果 |
+|---|---|
+| 支持切线交点有限且曲线不过远 | 生成采样圆角 |
+| 切线近似平行或交点退化 | 保留该角直线并记录 `junction-rounded-corner-fallback` |
+| 边界自相交或 connector 越界 | 使用 connector 凸包兜底；仍自相交则不发布路口面 |
+
+### 4. 必需测试
+
+- `npm run test:native-road`：普通 T / 十字路口的圆角顶点数、内收方向、内外人行道
+  曲线和 continuation 语义。
+- `npm run test:road-workbench`：工作台仍能加载 native 路口及人行道图层。
+- `npm run road:compile -- --config config/areas/nantaizi-lake-innovation-valley.json`。
+- `npm run road:check -- --config config/areas/nantaizi-lake-innovation-valley.json`。
+
+### 5. 错误与正确写法
+
+错误：先对路缘生成圆角，再把外侧边界按每个采样点线性平移；这会导致内外曲率不同，
+在人行道角落留下不一致的折面。
+
+正确：对内侧和外侧分别用相同的道路边缘支持切线规则生成曲线，仅在外侧切线退化时
+使用确定性的偏移回退。
+
 ## Native 道路中心虚线
 
 ### 1. 范围与触发条件

.trellis/tasks/08-13-native-road-compiler/task.json

@@ -21,7 +21,8 @@
   "children": [
     "08-14-native-road-lane-markings",
     "08-17-native-road-control-markings",
-    "08-17-native-road-center-lines"
+    "08-17-native-road-center-lines",
+    "08-17-native-rounded-junctions"
   ],
   "parent": null,
   "relatedFiles": [],

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

@@ -0,0 +1,2 @@
+{"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

@@ -0,0 +1,18 @@
+# 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

@@ -0,0 +1,2 @@
+{"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

@@ -0,0 +1,7 @@
+# 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

@@ -0,0 +1,31 @@
+# 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

@@ -0,0 +1,26 @@
+{
+  "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": {}
+}