[v0.3] Fix macOS PTY disconnect monitoring

[owainlewis]

Parent: #160.

The syntax-highlighting PR #174 exposed a pre-existing terminal disconnect bug.
The monitor polls stdin with events = 0; on macOS a direct PTY probe returns no hangup for that mask after the controller closes, while POLLIN returns POLLIN | POLLHUP.
The editor often exits later through a failed write, which masks the broken monitor until startup takes longer than the one-second test deadline.
Diagnostic logs confirmed the monitor was alive and repeatedly returned zero after controller closure.
A separate observed ttyname_r failure can also silently disable the guard; use descriptor device identity to exclude /dev/tty without name lookup.

Acceptance criteria:

• The monitor detects a direct macOS PTY controller disconnect independently of editor rendering or input dispatch.
• Keep the existing one-second exit requirement.
• Ordinary readable input is neither consumed by the monitor nor allowed to cause a busy loop.
• Redirected stdin and the special /dev/tty descriptor retain their existing cleanup behavior.
• No test deadline increases, skips, or retries used to hide a failure.

Checks:

• Regression test with a real PTY proves the polling mask reports controller hangup and preserves unread input.
• Focused descriptor classification tests for real terminal, /dev/tty, and non-terminal inputs.
• Terminal integration suite, formatting, Clippy, release build, and a real shell-restoration smoke check.
• Fresh review and required GitHub checks before merge.

[owainlewis]

Implemented in #176, commit 78c3af3.
The real-PTY regression fails with the old poll mask and passes with the fix; it also proves a live unrelated child cannot retain the controller.
All 322 unit tests, eight locally available terminal integration cases, formatting, all-target Clippy, release build, and a shell-restoration smoke check pass.
The two redirected-input cases require GitHub CI because the local sandbox rejects them.
Independent review: Approve after fixing test descriptor inheritance.

[owainlewis]

Merged #176 as dfd7a82.
GitHub CI passed all 322 unit tests and all ten terminal integration tests, plus formatting, Clippy, and release build.
Independent review approved the final fix; the local release smoke confirmed terminal restoration and a usable shell.