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

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

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+# Native Road Centre Lines Design
+
+## Architecture
+
+The native compiler adds one independent polygon layer:
+
+```text
+canonical bidirectional segment + trimmed OSM centreline
+  -> repeated 2m x 0.25m dash polygons
+  -> native-road/layers/center_lines.geojson
+  -> Workbench / native Blender adapter / Cesium
+```
+
+It does not read osm2streets `center_lines.geojson`. The legacy layer is a
+visual baseline only.
+
+## Placement
+
+- A candidate must have exactly forward and backward native roads for the same
+  `segmentId`, neither be `highway=service`, and have a valid trimmed OSM
+  centreline.
+- Dashes use the legacy observed dimensions: 2m length, 0.25m width, with a
+  deterministic 2m gap. The first dash starts at a fixed segment-local offset
+  so rebuilds do not drift.
+- The line is trimmed with the same `junctionPlans` cutback used by native lane
+  centreline generation. Any dash intersecting a crosswalk or stop line is
+  omitted, preserving the control-marking priority already established.
+- One-way and service segments have no generated feature. A degenerate source
+  line produces a diagnostic rather than malformed geometry.
+
+## Contracts
+
+`layers/center_lines.geojson` is a Polygon FeatureCollection. Every feature
+has `native_id`, `segment_id`, `directional_road_ids`, `osm_way_ids`,
+`dash_index`, `dash_length_m`, `dash_gap_m`, `placement_rule`, and
+`provenance="native-road-center-line/v1"`.
+
+`compiled.json.layers.centerLines`, comparison count
+`nativeCenterLineFeatures`, and `build-area.js` required native records use the
+same filename. `catalog.NATIVE_ROAD_LAYERS` maps source `center_lines` to the
+existing `center_lines` material layer.
+
+The Workbench loads this source into the marking overlay, exposes a Chinese
+toggle, and identifies it as `道路中心虚线` on selection.
+
+## Compatibility
+
+The change is additive within `native-road/`; `--road-provider osm2streets`
+remains unaffected and is rollback. A native Blender build treats a missing
+layer as an input error rather than silently omitting it.