Closes#89.
- _disposeWebGLObserver() called from both trip path and onContextLoss (fixes the leak)
- Thresholds (200ms/3/30s/5s/7d) hoisted to WEBGL_FALLBACK in constants.js
- Pure evaluateWebGLLongTaskTrip() helper + 9 Playwright tests (test/webgl-fallback.test.ts, port 3166)