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.

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