Replaces the scroll event listener and `getBoundingClientRect` polling with `IntersectionObserver` to eliminate layout thrashing and improve scrolling performance.
- Removes `window.onscroll` handler in `docsView.js`.
- Implements `IntersectionObserver` with `rootMargin` to track active headings.
- Retains initial one-time scan for correct state on load.
- Adds benchmark `benchmarks/docs_scroll_spy.html` and `benchmarks/docs_scroll_spy_bench.js`.
Benchmark results:
- Legacy: ~0.065ms/call (200 calls) -> Total ~13ms CPU (sync layout thrashing)
- Optimized: ~0.028ms/call (32 calls) -> Total ~0.9ms CPU
- 14x improvement in CPU time for scroll handler logic.
Co-authored-by: PR0M3TH3AN <[email protected]>
