feat: add live V2X Cesium preview overlay

.trellis/tasks/08-24-v2x-amap-intersection-data/implement.md

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+# Implementation Plan: Live V2X Cesium Intersection View
+
+1. Read preview and pipeline specifications, then add a focused V2X preview
+   configuration. Keep secrets and private origins out of defaults and tracked
+   area configuration.
+2. Add browser-safe GCJ-02-to-WGS84 conversion and focused tests for source
+   coordinate labels and prevention of duplicate conversion. Preserve the current
+   compiler reference conversion behavior.
+3. Implement the V2X browser client: login, session token lifecycle, REST
+   envelope handling, capability errors, and signal WebSocket lifecycle.
+4. Extend the generated Cesium preview with V2X entities and operations controls
+   for links, devices, poles/configuration, metrics, and live signal state.
+5. Add a local development/proxy configuration example and deployment
+   documentation for V2X REST/WS proxying. Do not add an AMap key, SDK, or page.
+6. Add focused tests for HTML generation, configuration escaping, login/request
+   construction, coordinate conversion, source-layer diagnostics, and graceful
+   unavailable/auth states. Run existing preview and native-road regressions.
+7. Validate in a browser with a real V2X account and configured proxy: sign in,
+   inspect aligned native/V2X features, verify REST panels and signal updates,
+   then verify sign-out/token expiry behavior.
+
+## Validation Commands
+
+```bash
+node --check scripts/lib/area-preview.js
+node --check scripts/lib/cesium-preview.js
+npm run test:gaode-junction-reference
+npm run test:preview-assets
+npm run test:native-preview-traffic
+npm run test:traffic-signals
+npm run road:compile -- --config config/areas/fengshu-er-road.json
+npm run build:area -- --config config/areas/fengshu-er-road.json --stages preview
+```
+
+## Review Gates
+
+- Confirm no token, account, or private host is committed.
+- Verify V2X GCJ-02 coordinates convert once before Cesium use.
+- Verify the existing native WGS84/ENU scene contract remains unchanged.
+- Verify the current Cesium preview works without V2X configuration or sign-in.