feat: add native center line style overrides

.trellis/tasks/08-17-native-road-center-line-styles/design.md

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+# Native Road Centre-Line Style Overrides Design
+
+## Architecture
+
+```text
+Workbench selects a generated centre-line dash
+  -> segment_id identifies the logical road segment
+  -> staged center-line-style override
+  -> native-road-overrides.json
+  -> compileCenterLines resolves default or effective style
+  -> center_lines.geojson polygons with style fields
+  -> Workbench / Blender / Cesium
+```
+
+The authoritative edit is a new override kind, not a mutation of
+`center_lines.geojson`. It has a deterministic ID based on `segmentId`, so a
+later recompilation replaces the segment's style rather than accumulating
+records.
+
+## Override Contract
+
+```json
+{
+  "id": "道路中心线:segment:way/123/1",
+  "kind": "center-line-style",
+  "segmentId": "segment:way/123/1",
+  "color": "yellow",
+  "pattern": "dashed"
+}
+```
+
+`validateOverrides()` accepts only known model segment IDs and the finite
+string enums `yellow|white` and `dashed|solid`. A style override applies once
+to the paired forward/backward native roads for that segment. The existing
+schema version remains `native-road-overrides/v1` because this is an additive
+kind and old files remain valid.
+
+## Geometry and Material
+
+- Default remains yellow dashed: 2m dash, 2m gap, 0.25m width.
+- `solid` generates deterministic adjacent 2m pieces with no gap. Pieces still
+  undergo the same junction cutback and control-marking exclusion as dashed
+  lines; this avoids creating a solid polygon across an excluded crossing.
+- Style fields `color`, `pattern`, `dash_length_m`, `dash_gap_m`, and
+  `effective_style` are stored on every generated polygon.
+- Blender currently maps all `center_lines` to one yellow material, so the
+  native adapter must support a white centre-line material route without
+  changing legacy osm2streets layers. The route must preserve the existing
+  yellow material for default and yellow overrides.
+
+## Workbench UX
+
+Selecting a centre-line dash shows a compact Chinese style panel in the
+existing form area. A select control presents the four named choices. Choosing
+one stages an override; the existing 保存 / 保存并重新生成 actions remain the
+only persistence and generation actions. The panel also shows whether the
+style is default or overridden and identifies the native road segment.
+
+## Compatibility and Rollback
+
+Unedited segments generate byte-compatible geometry style defaults apart from
+the added style properties. Oneway/service filtering and control priority are
+unchanged. Selecting `--road-provider osm2streets` remains a full rollback.
+