Slate v2 Scroll-into-view Deferral

Supporting plan. For current queue and roadmap truth, see master-roadmap.md.

Goal

Close the last vague part of the UI/layout bucket by making the scroll-into-view deferral explicit and evidence-backed.

Result

• no stable legacy Playwright row exists for scroll-into-view
• the legacy example itself is annotated as a possible future test harness, not a durable replacement-family surface
• the family stays intentionally later for an evidence reason, not by inertia