Implements the C1 retention contract: preserve worktree `hiddenSinceMs` through a
background-measure window (so TTL/ranking stay honest), but re-park waits for a
full `coldParkDelayMs` cool-down after the measure ends. Without the cool-down,
every ~3s measure lease on a past-deadline worktree thrashes remount/reattach.
Core changes:
- Terminal.tsx: add measure clock (measuringTerminalWorktreeIdsRef) and post-measure
cool-down tracking (terminalWorktreeParkCooldownUntilRef); gate parking candidates
until cool-down expires.
- Extract snapshot replay choreography to shared terminal-snapshot-replay-paint.ts
(used by SSH reattach + daemon restore paths).
- Add SSH model snapshot timeout (750ms) with fallback to relay replay.
- Move cold-park recheck deadline logic to terminal-cold-park-recheck-deadlines.ts;
add cool-down deadline to scheduling.
- useTerminalTabColdParking: implement matching measure-clock contract with per-tab
cool-down gate to keep tab deadlines synced with worktree retention clock.
- Add resolveTerminalMountScrollbackRows() to demote new xterms under demoted
worktrees (pane births during demotion must take the demoted tier at create).
- Add kill switches: terminalSshViewParking, terminalHiddenWorktreeRetentionBudget,
terminalHiddenScrollbackDemotion.