Experiment with mutation-boundary anchor capture

[tlongwell-block]

Summary

• add a flag-gated diagnostic that removes computeAnchor's full DOM-row geometry walk from each scrolled-up scroll event
• retain cheap bottom-state tracking in the hot path
• capture the exact visible row once at the older-history fetch boundary so prepend restoration keeps its existing contract

Test plan

• desktop check (one existing unrelated useOptionalChain warning)
• desktop typecheck
• desktop unit suite (2,269 passing)
• desktop production build
• focused Playwright scroll-history tests: reflow, single-page-per-gesture, prepend stability (3/3)
• pre-push desktop/mobile/Tauri suites
• buzz-db unit suite cannot start locally: Rust 1.89 is below sqlx 0.9's Rust 1.94 requirement
• trackpad A/B in Tauri: Settings → Experiments → Mutation-boundary timeline anchoring

Context

This is intentionally a standalone reversible diagnostic after #1691 showed no perceptible change. The hypothesis is that, while scrolled up, scanning all accumulated rows from oldest forward and reading rects on every native scroll event stalls WebKit momentum.