Skip empty replaceChildren visual-tree work #559

Parent: #243. CSS/performance epic: #235.

Proven hot path

Pinned unchanged Code OSS has repeated zero-argument replaceChildren() clears in SCM history badges, Chat/transcripts, inline Chat, onboarding and other workbench paths. WebScene detached/marked dirty/recascaded/published even when the target already had no native children or text.

Scope

After WebIDL conversion and receiver resolution, return only for zero-argument replaceChildren() when native children and text content are both empty. Real detach/reinsert/populate/clear operations retain all mutation, custom-element, layout, selector and publication behavior.

Acceptance

• Focused source and regression authored.
• git diff --check passes.
• 4,096 connected plus 4,096 detached empty clears add zero layout/publications, recascade requests, lifecycle callbacks or nodes.
• 512 custom rows prove exact detach/reinsert/populate/real-clear/restore callbacks, visual publication, selector style, identity and low-memory reclamation.
• Pinned Code OSS A/B/ABBA, browser/package, pixel, physical-performance and memory gates run later.

No build, test, benchmark or formatter was executed.