restore, step 1: layout snapshots and dingger restore

the daemon saves the tmux layout when it changes: sessions, windows
(index, name, automatic-rename, layout, size, zoom, active), and panes
(directory, active, agent). a new history file for layout changes, a
rewrite for details, the last 50 kept, nothing saved while tmux is down.
when a different server runs (or none), the old server's last snapshot is
pinned as the restore candidate before the new server's snapshots start.

dingger restore rebuilds missing sessions by tmux id: panes split from the
newest so they keep their order, the saved layout applied at the saved
size, then the size released so windows follow clients again. a bare
session of the same name (a terminal's `new-session -As 0`) is replaced,
its clients switched over first; used sessions are kept. --dry-run,
--list, --from, --session.

e2e step 16 kills a server with a client attached like ghostty's, restarts
it, restores, and compares every window and pane; splitting the wrong
pane, not replacing bare sessions, not pinning, or keeping the saved size
each fail it. e2e keeps its state in its temp dir.

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