Add signal handling to FUSE examples

[Copilot]

The FUSE examples need proper signal handling to ensure graceful unmounting when terminated with signals like SIGINT (Ctrl+C), SIGTERM, or SIGHUP.

Changes

Applied signal handling pattern from basic.rs to remaining examples:

• hello.rs - Added signal handler thread
• heartbeat.rs - Added signal handler alongside heartbeat thread
• heartbeat_entry.rs - Added signal handler alongside heartbeat thread
• poll.rs - Added signal handler thread

Each example now:

1. Spawns a dedicated thread listening for SIGTERM, SIGHUP, SIGINT
2. Calls mount.unmount() on signal reception
3. Uses thread::scope to ensure proper cleanup

thread::scope(|scope| -> Result<()> {
    let mut signals = Signals::new([SIGTERM, SIGHUP, SIGINT])?;
    scope.spawn(move || {
        if let Some(_sig) = signals.forever().next() {
            let _ = mount.unmount();
        }
    });
    
    // Main FUSE request loop...
})?;

Removed explicit mount.unmount() calls at end of main since the signal handler now manages unmounting.

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