chore(task): plan native-only pipeline replacement

.trellis/tasks/08-18-native-only-pipeline-replacement/implement.md

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+# Implementation Plan
+
+1. Add native provider/default-stage contracts and explicit legacy provider routing.
+2. Make package publication and stage manifests consume native runtime artifacts.
+3. Replace preview's hard dependency on osm2streets lane/network/intersection files with a native
+   route adapter and optional-route behavior.
+4. Audit Blender, Cesium, preview, diagnostics, and area config for native-path legacy reads; retain
+   only the Workbench's optional, clearly labeled osm2streets reference layer.
+5. Update README/config/spec documentation and add dependency-boundary regression tests.
+6. Run native-only tests with legacy directories absent, then run focused legacy compatibility tests.
+7. Run area compile/check plus available Blender/Cesium/preview validation; record any environment-only
+   limitation without weakening native contracts.
+
+## Validation
+
+```bash
+npm run test:native-road
+npm run test:traffic-signals
+npm run test:road-workbench
+npm run test:preview-assets
+npm run road:compile -- --config config/areas/nantaizi-lake-innovation-valley.json
+npm run road:check -- --config config/areas/nantaizi-lake-innovation-valley.json
+node --check scripts/build-area.js
+```
+
+The final gate must also exercise a native-only config/output directory without QGIS/osm2streets
+intermediates and assert package/runtime/preview outputs.