# Native road center lines

## Goal

Restore the existing osm2streets-style road-centre dashed markings in the
native-road provider before treating native output as ready for broader quality
gates or new-area validation.

## Confirmed Facts

- Nantaizi's existing `osm2streets_web_out/center_lines.geojson` has 885
  `type="center line"` polygons. Its sampled dash geometry is approximately
  2.00m long by 0.25m wide.
- The legacy extraction excludes centre lines that overlap crosswalk zones and
  that fall on service driving polygons (`build-osm2streets-qgis.js:474-480`).
- Native output currently has only same-direction `lane_separators`; it has no
  `center_lines.geojson`, so a bidirectional road's directional carriageways
  lack their visual and semantic divider.
- Existing Blender already owns a `center_lines` / `Center Line` material
  layer. Native must adapt to it rather than creating a second scene registry.

## Requirements

- R1: For supported non-service, bidirectional native road segments, emit a
  `center_lines.geojson` dashed centre divider derived from the canonical OSM
  centreline and the segment's two directional carriageways.
- R2: Match the established visual baseline: yellow 0.25m-wide, 2m-long
  dashes, clipped away from ordinary junction cutbacks and control markings.
- R3: Preserve source traceability: each dash records its native segment,
  source OSM way, involved directional roads, placement interval/rule, and a
  dedicated provenance value.
- R4: Add the layer to compilation records, Road Workbench display/selection
  in Chinese, and the existing Blender/Cesium `center_lines` material path.
- R5: Do not render a centre divider on one-way or `highway=service` segments;
  report invalid geometry instead of fabricating a divider.

## Acceptance Criteria

- [ ] Native Nantaizi output contains a valid `layers/center_lines.geojson`
  with source-traceable 2m x 0.25m dashed polygons.
- [ ] No native centre dash intersects a generated crosswalk or vehicle stop
  line, and no dash reaches into the supported junction surface cutback.
- [ ] The Workbench can toggle and select centre lines, presenting their
  source and rule in Chinese rather than calling them lane separators.
- [ ] Native `blender,cesium,preview` consumes the layer using the existing
  `center_lines` material and does not run `package`.
- [ ] Tests cover two-way generation, one-way/service skips, control-marking
  avoidance, provenance, output contract, and Workbench wiring.

## Out Of Scope

- Centre-line editing overrides, solid/double-centre-line semantics, arbitrary
  OSM `overtaking` interpretation, lane colouring changes, and other areas.

## Key Decision

This task deliberately matches the existing stable visual layer first. It does
not claim to infer a country-wide legal marking taxonomy from sparse OSM tags.