chore(task): archive 08-18-native-only-pipeline-replacement

.trellis/tasks/archive/2026-08/08-18-native-only-pipeline-replacement/design.md

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+# Design
+
+## Provider Boundary
+
+`build-area.js` will resolve a native provider by default. Native stages consume only OSM, native
+road overrides, and `native-road/` outputs. Legacy `intermediates`, `reimport`, and osm2streets
+Blender input remain behind explicit legacy selection and keep their existing output contract.
+
+## Native Data Flow
+
+```text
+OSM + native-road-overrides.json
+  -> compile-native-roads.js
+  -> native-road/layers/*.geojson + compiled.json + traffic-signals.json
+  -> Blender generate_scene.py --native-road --traffic-signals
+  -> .blend
+  -> Cesium/package GLB + manifest + runtime/traffic-signals.json
+  -> preview HTML + optional native vehicle route
+```
+
+The native compiler is the sole authority for road geometry, lane semantics, intersection surfaces,
+stop lines, and signal runtime. No native stage reads `geojsonDir` or legacy assembly files.
+
+## Preview Route
+
+Add a native route adapter that derives route segments from `compiled.json` model roads/endpoints and
+native lane/connector geometry. If the native route cannot be built for a valid area, preview remains
+usable without vehicles and records a warning; missing legacy osm2streets lane polygons is never a
+hard failure on the native path.
+
+## Package Runtime
+
+The package stage will copy the native-road signal runtime into its staging runtime directory and
+declare it in the package manifest. Preview receives the package-relative runtime URI. Legacy signal
+runtime publication remains conditional on the legacy provider.
+
+## Configuration And Compatibility
+
+`roadProvider` defaults to `native`; native stages are the default stage set. QGIS config fields and
+legacy stage aliases remain accepted for explicit migration/debug commands, but native manifests and
+docs do not claim them as inputs.
+
+The Workbench may continue to read an existing `geojsonDir` only for an explicitly labeled reference
+layer and comparison counters. Those reads are optional, isolated from native compile state, and must
+never become required inputs for `/api/state`, native editing, package publication, or preview.
+
+## Rollback
+
+The original QGIS/osm2streets branch baseline is tagged
+`qgis-original-baseline-20260818` on `main`. Reverting the native-only work can therefore use the tag
+or switch to `main`; no destructive cleanup of legacy scripts is required.