chore(task): archive 08-18-native-traffic-signal-parity

.trellis/tasks/archive/2026-08/08-18-native-traffic-signal-parity/design.md

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+# Design
+
+## Source Of Truth And Migration
+
+The native compiler owns a versioned area-local signal model and override
+artifact. It is generated from OSM controls and native geometry inputs, then
+edited by the Workbench without QGIS. The existing
+`traffic_signal_assemblies.geojson` is a migration adapter: it can be imported
+into the native model and exported for legacy QGIS/reimport workflows, but a
+native compile never requires it to exist.
+
+The existing `buildTrafficSignalFeatures()` and validation functions remain the
+compatibility implementation for initial generation and import/export. Imported
+features retain their legacy `signal_uid` where valid; newly generated native
+features use the same deterministic identity rule so downstream runtime IDs do
+not fork.
+
+## Workbench API And Editing
+
+`GET /api/state` adds the normalized native signal model, source control
+metadata, migration provenance, and derived runtime signal records. A signal
+edit is represented as an atomic replacement of the validated native signal
+override artifact through a dedicated signal save endpoint. A separate import
+or export action handles the legacy QGIS collection; road overrides remain in
+their existing file and schema.
+
+The browser uses stable `signal_uid` values. It supports:
+
+- generate: choose an OSM traffic-signal control and arm, then create the
+  deterministic assembly using the existing generator contract;
+- move: update the Point coordinates while retaining stop-line/source fields;
+- rotate: update `heading_deg` with normalized degrees;
+- delete: remove the assembly from the editable collection;
+- edit enabled state, display ID, mast reach, z offset, and phase group.
+
+Every save validates uniqueness, identity, finite geometry, source references,
+and field ranges before an atomic write. Deleted features are absent from the
+runtime output; disabled features remain in the editable/QGIS layer but are
+omitted by `buildTrafficSignalsFromFeatures()`.
+
+## Delivery Flow
+
+The native road compile result includes signal assemblies and derived runtime
+metadata without adding them to road geometry layers. Native Blender/Cesium
+stages consume `traffic_signals.json` and dynamic GLB inputs generated directly
+from the native model. The legacy QGIS adapter may materialize the old GeoJSON,
+but it is not in the native build's critical path.
+
+## Compatibility And Rollback
+
+QGIS reimport continues to read/export the compatibility GeoJSON while the
+legacy path remains unchanged. If native signal editing fails validation, the
+previous atomic native override remains in place and the user receives a
+field-level error. Rollback is selecting the legacy provider or exporting the
+last native state to the QGIS adapter.