Snapshot Diff: Use flow-renderer persistentHighlight instead of auto-clearing highlights

[andypalmi]

Description

The flow-renderer's compare() auto-clears node and tab glow highlights after 10 seconds. In the snapshot diff viewer, the user navigates between changes and needs highlights to persist until moving to the next one.

Changes

• Pass persistentHighlight: true to flowRenderer.compare()
• Store clearHighlight reference from the comparison result for manual cleanup (falls back to no-op on older renderer versions)

Dependencies

• Add persistentHighlight option and clearHighlight() API flow-renderer#91

Parent issue

Part of #7062