perf(terminal): optimistic close — instant pane removal, busy check off-thread

[matej21]

Problem

Closing a terminal had a perceptible few-hundred-ms lag on macOS (not reproducible on Linux).

The grace-period soft-close busy check ran synchronously on the GPUI thread, before the pane was removed from the layout. On macOS that check forks subprocesses:

• get_foreground_shell_pid → spawns tmux list-panes (tmux backend) or lsof ×2 (dtach backend)
• has_child_processes → spawns pgrep -P

On Linux these are sub-millisecond /proc reads, so the lag was macOS-only. Each fork+exec on macOS is expensive (dyld, AMFI/codesign), and they sat directly on the close keypath.

Fix

Make every interactive close optimistic:

1. Eject the pane from the layout immediately (PTY stays alive) — instant, no subprocess work on the GPUI thread.
2. Run the busy probe off the GPUI thread via smol::unblock.
3. When it returns: idle → kill the PTY now (no toast); busy → show the undo toast + grace timer (unchanged behaviour).

The end state is identical to before — only the ordering changed — and the UI now updates instantly regardless of session backend (tmux/dtach/plain). Applies to both single (CloseTerminal) and multi (CloseTerminals) close.

Changes

• okena-workspace: Workspace::decide_pending_close → PendingDecision { Raced, Finalized, KeepForUndo }. Pure decision logic, unit-tested; the UI layer only handles the toast.
• okena-app: soft_close::try_begin → begin (optimistic); extracted build_soft_close_toast. A stable toast id is reserved up front so the toast is posted later only if the terminal turns out busy.
• okena-state: Toast::with_id.

Races

Unchanged. If the shell exits during the probe or grace window, the existing exit-path cancel_pending_close / reap_restored_close handle it, and decide_pending_close no-ops when the pending record is already gone (Raced).

Tests

• decide_keeps_busy_terminal_for_undo, decide_finalizes_idle_terminal, decide_is_noop_when_pending_already_gone (okena-workspace).
• Full workspace build, existing soft-close suite, and clippy all green.

Follow-up PR replaces the macOS subprocess introspection (pgrep/lsof) with libproc syscalls — orthogonal perf/battery win.

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