Re-measure document-root layout and layerization after the ECS redraw fix

## Problem

The #15977 trace shows frequent document-root layout and material layerization tails, but duplicate redraws confound their independent cost. They must be re-measured after the feedback-loop fix rather than silently treated as resolved.

## Goal

Determine whether layout scope or layerization remains a material post-fix bottleneck.

## Proposed Solution

Repeat the exact workload after the redraw fix, retain style/layout/paint/layer events, and isolate DOM invalidation scope and layerization cadence.

## Acceptance Criteria

- Before/after runs use equal workflow, events, viewport, and browser configuration.
- Document-root invalidations and layerization p95 are reported.
- Any remaining material cause is reduced to a detector and bounded fix.
- Non-material findings are closed with measured evidence.