chore(task): archive 08-17-native-road-center-lines

.trellis/tasks/archive/2026-08/08-17-native-road-center-lines/prd.md

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+# 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.