feat: add native lane separator style overrides

.trellis/tasks/08-17-native-lane-separator-styles/design.md

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+# Native Lane Separator Style Overrides Design
+
+## Contract
+
+The override is identified by the stable `roadId` plus the adjacent lane pair:
+
+```json
+{
+  "id": "车道分隔线:road:way/123:forward:1-2",
+  "kind": "lane-separator-style",
+  "roadId": "road:way/123:forward",
+  "leftLaneIndex": 1,
+  "rightLaneIndex": 2,
+  "color": "white",
+  "pattern": "dashed"
+}
+```
+
+The same `yellow|white` and `dashed|solid` enums are reused. A default is
+white dashed. The generated Polygon stores `color`, `pattern` and
+`effective_style`; source IDs remain unchanged.
+
+## Rendering
+
+The existing lane separator polygon represents the whole lane-pair path.
+`solid` retains it as one continuous feature. `dashed` samples regular dash
+polygons along the shared centreline with deterministic spacing and preserves
+junction cutback. White uses the existing `lane_separators` material; yellow
+uses a native-only yellow material route. Workbench separates lane separators
+from turn arrows so their styles remain selectable and visible.
+
+## Compatibility
+
+No override means current visual default. Road edges, curbs and centre lines
+are untouched. Existing `native-road-overrides/v1` files remain valid.