fix(ssh): exit closes the tab instead of reconnecting (#538)

[kipavy]

Fixes #538.

Causes

1. Close decided on EOF. channel_io emitted ssh-closed on the server's EOF, before the exit-status that marks a deliberate exit. Traced on a real RouterOS 7.23.7 (CHR): /quit sends EOF → exit-status → CLOSE, so it read as a dropped link and reconnected.
2. Persistence silently downgraded. The backend drops persist on Windows hosts (and when the wrapper exceeds dropbear's limit), but the frontend still treated the tab as persistent, skipped the clean-exit path and reconnected to a plain shell — with no attach probe to stop it.

Fix

• Only CLOSE (or the end of the stream) settles the channel; an exit-status/exit-signal anywhere before it counts (CloseWatch).
• A wrapped persistent session's clean exit probes the multiplexer on the still-open connection: gone → ended, present (detach) → reconnect as before. The attach-time probe now shares persistent_session_present.
• Frontend: remoteExit now means "ended" and closes the tab regardless of persist; persistent tabs are also tombstoned. Linux persistent exit no longer shows the 2–3 s reconnect checklist before closing.

Verification

Live, private headless container with this build:

Case	Result
RouterOS /quit, persist off	tab gone in 0.5 s
same, pre-fix binary (control)	connecting → reconnected at 3 s (bug reproduced)
Linux exit, persist off	gone in 0.5 s
Linux exit, persist on	gone in 0.5 s, no reconnect
Linux tmux detach, persist on	connecting → reconnected in 1.7 s (unchanged)

Unit tests for both message orders and a drop; reconnectBackoff tests; tsc; cargo fmt --check; cargo clippy --all-targets -D warnings.

Not run on a Windows host: relies on Windows OpenSSH sending exit-status before CLOSE (shared OpenSSH session code). Cause 2 is fixed regardless.