chore(task): archive 08-18-native-preview-traffic-simulation

.trellis/tasks/archive/2026-08/08-18-native-preview-traffic-simulation/design.md

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+# Design
+
+## Authority And Boundaries
+
+The native compiler remains the only road authority. The existing traffic-signal runtime remains the
+only signal authority. The new feature adds a route/simulation layer beside those artifacts; it does
+not alter signal geometry, phase grouping, or Blender static assets.
+
+```text
+native-road/compiled.json + native-road/layers/*
+  -> native route compiler
+  -> _preview/<area>-traffic-simulation.json
+  -> Cesium preview runtime
+
+package/runtime/traffic-signals.json
+  -> signal phase clock + stop events
+```
+
+The descriptor is intentionally package-adjacent, not embedded in the GLB. A consumer can copy the
+package and descriptor to another platform and reproduce the simulation without running Node.
+
+## Descriptor Contract
+
+Schema: `native-preview-traffic-simulation/v1`.
+
+Required top-level fields:
+
+- `areaId`, `coordinateSystem` (`WGS84` route coordinates, `ENU` model placement), `generatedAt`
+- `source` with relative artifact references and SHA-256 records
+- `settings` with speed, acceleration, deceleration, reaction time, vehicle length, and minimum gap
+- `routes[]` with ordered WGS84 coordinates, cumulative distances, connector/maneuver IDs, and
+  `stops[]`
+- `signals[]` copied by stable `signal_uid` from the runtime contract, including phase group and
+  stop-line association; no regenerated pose fields
+- `migration` describing schema version, coordinate conversion, update-loop expectations, and
+  compatibility notes
+
+The descriptor is deterministic for the same OSM, overrides, native compiler version, and settings.
+
+## Route Construction
+
+Use native directional roads and published connector geometry. Build a small number of deterministic
+closed demonstration routes that cross real connectors and retain lane/road IDs. Route generation
+must reject routes with missing geometry or disabled connectors and write structured diagnostics.
+Native stop lines are matched by `road_id`/`lane_id` where available, with a bounded geometric
+fallback recorded in the stop record. No fixed-width or legacy polygon fallback is allowed.
+
+## Browser Simulation
+
+Keep the existing signal phase functions and `signalData` payload. Replace the current per-vehicle
+independent distance advance with a shared simulation state:
+
+- each vehicle has route distance, speed, desired speed, status, and active stop reason;
+- signal stop constraints are evaluated before the native stop line for red/yellow phases;
+- leader constraints are evaluated on the same route and wrap around the loop;
+- acceleration/deceleration clamps advance toward the minimum of desired speed, signal limit, and
+  leader-safe speed;
+- vehicle position and orientation remain Cesium callback properties, with no per-frame allocation
+  of heavyweight Cesium objects;
+- incident-card behavior remains local to each vehicle and must not mutate the descriptor or package.
+
+Diagnostics expose route count, signal count, stopped vehicles, and queue length in the existing
+preview diagnostics panel.
+
+## Migration And Rollback
+
+Document the descriptor, runtime signal schema, coordinate conversion, and a reference update loop in
+`docs/native-preview-traffic-simulation.md`. Include a JSON example and a consumer checklist. Legacy
+preview route files remain readable only through the existing legacy provider path; native generation
+must not depend on them. Removing the new descriptor restores the existing no-cruise native preview.