# 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.