fix(app): restore scroll position when navigating between parent and child sessions - #42809

AI code review — automated review for reference; please use your judgment.

Review of the scroll-position restoration fix:

1. packages/app/src/pages/session.tsx (~L1509) / packages/app/src/pages/session/timeline/message-timeline.tsx (~L273) — restoration mode has no exit condition: isRestoring is fixed at setup, and the live checks skip anchoring whenever a cached scrollOffset exists — so after returning to a session, bottom-anchoring stays suppressed for the whole mount, including forceScrollToBottom (~L1989): send a message in a restored session and the view will not follow the new reply — clear the cached offset once it has been consumed (first post-mount observation) or on a user scroll gesture (props.hasScrollGesture()) so normal anchor behavior resumes.
2. packages/app/src/pages/session/timeline/message-timeline.tsx (~L424) — the cached offset is applied verbatim as initialOffset even though content may have changed while away (new child-session output, reverted messages shrinking the list) — an offset beyond the new extent either clamps oddly or lands far from the same semantic position — validate/clamp the restored offset against current total size (or fall back to anchor-bottom when measurements mismatch beyond a tolerance).
3. packages/app/src/pages/session.tsx (~L1509, ~L1578, ~L1989) — timelineCache.get(sessionKey())?.scrollOffset !== undefined is spelled out three times plus the timeline-side copy — extract one hasCachedScroll(sessionKey) helper next to timelineCache so the "restoring?" definition lives in exactly one place.
4. Test coverage — the decision matrix (cold mount anchor-bottom / restore-with-cache / restore-then-send-message) is pure enough to unit test if the helpers from points 1 and 3 are extracted — worth doing before this interaction surface grows more branches.

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