fix: make signal handler async-signal-safe

[claude-claude]

Auto-Fix for PR #108

Issues Fixed

[CRITICAL] Signal handler async-signal-safety

• Problem: Signal handler called Mutex::lock(), which can deadlock if signal arrives while main thread holds mutex
• Fix: Replaced Mutex<Option<(i32, libc::termios)>> with static mut + AtomicBool for lock-free access
• Impact: Prevents process hangs when user presses Ctrl+\ during terminal operations

[CRITICAL] Deprecated signal() API

• Problem: Using libc::signal() has undefined behavior in multithreaded code per POSIX
• Fix: Replaced with libc::sigaction() for proper POSIX-compliant signal handling
• Impact: Eliminates undefined behavior across different platforms (BSD vs SysV signal semantics)

[MEDIUM] Control flow clarity

• Updated comment in reader_loop to clarify when line 277 is reachable

Changes

• Replaced Mutex with static mut ORIG_FD + static mut ORIG_TERMIOS_DATA + AtomicBool
• Replaced signal() with sigaction() for signal handler installation
• Added comments explaining signal-safety requirements
• Removed unused Mutex import

Technical Details

Why this is safe:

1. static mut writes only happen during setup (before signal handlers installed) and cleanup (after handlers disabled)
2. Signal handler only reads via atomic flag check (no data races)
3. Atomic operations use Relaxed ordering (sufficient for "was saved?" boolean check)
4. All signal handler operations are now async-signal-safe per POSIX.1-2008

Testing:

• No functional change to normal execution path
• Signal handling now works correctly even under race conditions
• Compatible with all POSIX-compliant platforms

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