Client: bound history memory for visited threads #525

Summary

Bound client-side history memory. A client keeps the records of every thread it has visited for its whole lifetime, and loading older pages re-folds everything already held.

Current behaviour

• session_records in crates/ui/src/store/mod.rs is a map by thread id that is never pruned; nothing in crates/ui/src removes entries.
• Each earlier page re-folds all held records (crates/ui/src/store/history.rs), so scrolling up a long thread costs roughly the square of the pages loaded (inferred).
• Prefetch keeps 6 screens above the viewport (HISTORY_WINDOW_SCREENS).

Proposal

• Release a thread's records when it has not been viewed for a while (T3 Code uses a 5-minute idle TTL for thread snapshots), keeping the currently open thread and threads with running turns.
• Fold incrementally, or fold per turn segment, so prepending a page does not re-fold the whole held window.
• This matters most on phone clients and remote sessions.