feat: add native road marking semantics

.trellis/tasks/08-17-native-road-marking-semantics/prd.md

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+# Native road marking semantics
+
+## Goal
+
+Finish the native road marking model so the Nantaizi Road Workbench can
+persist meaningful centre-line and road-edge-line styles, regenerate native
+geometry deterministically, and deliver the same result to Blender/Cesium.
+
+## Requirements
+
+- Add an explicit double-yellow-solid centre-line semantic. It must not be a
+  generic double-line switch that permits invalid colour/pattern combinations.
+- Keep all marking edits area-local in `native-road-overrides.json`; generated
+  GeoJSON remains derived output.
+- Make road edge lines selectable and style-editable in the Chinese Road
+  Workbench, alongside existing centre lines and lane separators.
+- Preserve the default Nantaizi output unless a user supplies an override.
+- Native geometry must remain independent of osm2streets render geometry.
+
+## Acceptance Criteria
+
+- [ ] A `center-line-style` override with `double: true`, `yellow`, and
+  `solid` emits two offset centre-line polygons with an explicit effective
+  style, while invalid double combinations are rejected.
+- [ ] An `edge-line-style` override targets one directional road side and
+  supports white/yellow plus solid/dashed styles.
+- [ ] The Workbench exposes Chinese selection feedback, stages the correct
+  override payload, and only shows the double-yellow choice for centre lines.
+- [ ] Nantaizi native compile/check, Workbench/native tests, Blender pure
+  tests, and native Blender/Cesium/preview build pass.
+
+## 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`.