feat: add native preview traffic simulation

.trellis/tasks/08-18-native-preview-traffic-simulation/implement.md

@@ -0,0 +1,37 @@
+# Implementation Plan
+
+1. Add a native route compiler module that consumes `native-road/compiled.json` and native layers,
+   emits `native-preview-traffic-simulation/v1`, and records source hashes and migration metadata.
+2. Wire native preview generation to write the descriptor and keep legacy osm2streets route
+   generation unchanged.
+3. Extend the browser preview with shared deterministic vehicle state, signal stop constraints,
+   leader following, queue diagnostics, and graceful empty-route behavior.
+4. Preserve the existing signal phase, `signal_uid`, pose, dynamic lens, countdown, disabled-signal,
+   and incident-card contracts; add focused regression tests for each boundary.
+5. Add `docs/native-preview-traffic-simulation.md` with schema, example, coordinate rules, update
+   loop, migration checklist, and rollback notes.
+6. Run focused Node tests, native compile/check, a native build with no legacy directory, and browser
+   preview validation in the available Blender/Cesium environment.
+
+## Validation Commands
+
+```bash
+node --check scripts/lib/native-preview-traffic-simulation.js
+node --check scripts/build-area.js
+npm run test:native-road
+npm run test:traffic-signals
+npm run test:preview-assets
+npm run test:native-preview-traffic
+npm run road:compile -- --config config/areas/nantaizi-lake-innovation-valley.json
+npm run road:check -- --config config/areas/nantaizi-lake-innovation-valley.json
+npm run build:area -- --config config/areas/nantaizi-lake-innovation-valley.json --stages blender,cesium,compress,package,preview
+```
+
+## Risk Controls
+
+- Do not change `scripts/lib/traffic-signals.js` geometry or phase semantics unless a test proves a
+  migration bug; prefer adapter code.
+- Do not add route data to the static package manifest; it belongs beside `_preview/` and is optional.
+- If native route generation produces no valid route, emit a valid descriptor with diagnostics and
+  keep preview usable.
+- Keep all route and simulation math unit-testable without Cesium.