ShadowDOM support benchmarks

Deterministic fixtures for manually measuring Shadow DOM discovery, lifecycle handling, mutation processing, and snapshot transport.

• Light DOM baselineMeasure a predictable light-DOM fixture for comparison.
• Nested Shadow DOM discoveryExercise discovery through nested open Shadow DOM roots.
• Mutation churnMeasure repeated DOM mutations within observed Shadow DOM.
• Shadow-root lifecycleExercise attaching, detaching, and reattaching Shadow DOM hosts.
• Snapshot sizeGenerate snapshots at controlled sizes, including large payloads.
• Repeatability checkRun the scenarios repeatedly and compare their reported metrics.