chore(task): plan native-only pipeline replacement

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

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+# Complete native-only pipeline replacement
+
+## Goal
+
+Make the native road compiler the primary and complete production path. A normal build of an
+area must generate Blender, Cesium/package, and preview outputs directly from OSM plus native
+artifacts without installing or running QGIS, GDAL, or osm2streets.
+
+The existing QGIS/osm2streets workflow remains available only as an explicit legacy/debug path and
+an optional Workbench reference layer. It is protected by the `qgis-original-baseline-20260818` tag
+on `main`.
+
+## Confirmed Current Gaps
+
+- `normalizeAreaConfig()` defaults `stages.intermediates` to true and `blender.roadProvider` to
+  `osm2streets`.
+- `build-area.js` sends `intermediates` to `build-osm2streets-qgis.js` and `reimport` to GDAL/QGIS.
+- Native Blender input exists, but `preview` still requires `osm2streets_web_out/lane_polygons.geojson`,
+  `network.json`, and `intersection_surface.geojson` for vehicle-route generation.
+- Package/runtime publication still copies `outputs.<area>.trafficSignals` from the legacy GeoJSON
+  directory instead of the native-road runtime artifact.
+- README, config templates, stage names, and manifest descriptions still present QGIS as the normal
+  workflow.
+
+## Requirements
+
+- R1: Native is the default road provider and default build stages do not invoke QGIS, GDAL, or
+  osm2streets.
+- R2: Native build generates all required road, lane, marking, junction, stop-line, and traffic-signal
+  inputs consumed by Blender and preview; no native stage reads `osm2streets_web_out`.
+- R3: Native package publication copies runtime traffic signals and any other runtime descriptors
+  from native-road outputs, with stable package-relative URIs.
+- R4: Native preview route generation uses native lane/road topology and native intersection geometry,
+  or explicitly omits the optional vehicle route when native route data is unavailable; it must not
+  fail because legacy osm2streets files are absent.
+- R5: QGIS/osm2streets stages remain callable only through an explicit legacy provider/stage selection
+  and are not part of native defaults or native manifests. Workbench may display their existing
+  GeoJSON as a clearly labeled reference/comparison layer, but native editing, compile, package, and
+  preview results must not depend on it.
+- R6: Documentation and config templates describe native-only as the primary workflow and clearly
+  mark the legacy path as transitional/debug-only.
+
+## Acceptance Criteria
+
+- [ ] A clean native-only area build succeeds with QGIS absent and no `osm2streets-js-node` runtime
+  call, producing `.blend`, package manifest/GLB, runtime traffic signals, and preview HTML.
+- [ ] Native preview opens when `osm2streets_web_out/` is absent; optional vehicle cruise either uses
+  native route data or is omitted with a visible non-fatal diagnostic.
+- [ ] Native package manifest contains runtime traffic-signal anchors sourced from
+  `native-road/traffic-signals.json`; disabled signals remain absent from runtime.
+- [ ] Workbench edits survive save, native recompile, package publication, and preview reload without
+  QGIS or osm2streets files.
+- [ ] Explicit legacy QGIS/osm2streets commands still pass their existing focused tests, but no longer
+  run when using native defaults.
+- [ ] Workbench remains usable when the optional osm2streets reference directory is absent; only the
+  reference layer/comparison counters degrade to unavailable.
+- [ ] Tests assert the native-only dependency boundary and the QGIS baseline tag is documented.
+
+## Out Of Scope
+
+- Reimplementing QGIS editing features in the native compiler beyond existing Workbench controls.
+- Deleting legacy scripts or historical output files in this task; they remain rollback/debug tooling.
+- Changing Blender/Cesium visual semantics unrelated to removing the input dependency.