chore(task): archive 08-17-native-lane-separator-styles

.trellis/tasks/archive/2026-08/08-17-native-lane-separator-styles/prd.md

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+# Native lane separator style overrides
+
+## Goal
+
+Make native same-direction lane separators selectable and persistently editable
+in Road Workbench, using the style override pattern proven for centre lines.
+
+## Confirmed Facts
+
+- `compileLaneMarkings()` writes one continuous `0.12m` Polygon separator
+  between each adjacent lane pair, identified by `road_id`, `left_lane_index`
+  and `right_lane_index`.
+- Lane separators currently have fixed light marking material and are displayed
+  together with turn arrows in Workbench.
+- `native-road-overrides.json` already supports validated, persistent centre
+  line styles by logical native segment. The new type must not alter road-edge,
+  curb or sidewalk geometry.
+
+## Initial Requirements
+
+- Selecting a lane separator must show its lane-pair source and a Chinese
+  style editor.
+- The first style catalog should mirror centre lines: white/yellow and
+  dashed/solid.
+- The effective style must be generated into GeoJSON, visible in Workbench,
+  and consumed by native Blender/Cesium.
+- Default output must remain the current white dashed-style separator.
+
+## Out Of Scope
+
+- Road-edge lines, curbs, sidewalks, double lines, partial ranges, lane-specific
+  legal restrictions and osm2streets layers.
+
+## Key Decision
+
+Each override applies only to the selected adjacent lane pair. This preserves
+separate marking semantics on roads with three or more lanes.
+
+## Acceptance Criteria
+
+- [ ] Selecting a separator exposes a Chinese style editor for its adjacent
+  lane pair; choosing a style stages it automatically.
+- [ ] Save and regeneration preserve the per-pair style across reloads while
+  unedited separators retain the default.
+- [ ] GeoJSON, Workbench, Blender and Cesium show the same effective style.
+- [ ] Tests cover validation, defaults, per-pair isolation, automatic staging
+  and native scene consumption.