feat: add native center line style overrides

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

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+# Implementation Plan
+
+1. Add validated `center-line-style` overrides and a helper which resolves the
+   effective default/override style by segment ID.
+2. Generate yellow/white and dashed/solid centre-line geometry while retaining
+   cutback and control-marking exclusion behaviour; publish effective style
+   properties.
+3. Extend the native Blender layer adapter/material handling to distinguish
+   white from yellow centre-line features without affecting legacy layers.
+4. Add Workbench style controls, staging, Chinese selection evidence, and
+   immediate regenerated-layer display.
+5. Add focused compiler, override, Workbench, Blender catalog, and build-stage
+   tests; compile/check Nantaizi and build Blender/Cesium/preview without the
+   package stage.
+
+## Validation
+
+```bash
+npm run test:native-road
+npm run test:road-workbench
+python3 -m unittest discover blender/tests
+npm run test:build-stages
+npm run road:compile -- --config config/areas/nantaizi-lake-innovation-valley.json
+npm run road:check -- --config config/areas/nantaizi-lake-innovation-valley.json
+npm run build:area -- --config config/areas/nantaizi-lake-innovation-valley.json --stages blender,cesium,preview --road-provider native
+```
+
+## Rollback
+
+Remove the override records or choose `--road-provider osm2streets`; no legacy
+output is modified.