fix: instant toggle exit via SIGUSR1 poll interruption

Remove SA_RESTART from the signal handler so SIGUSR1 interrupts the
blocking poll() call with EINTR. Handle EINTR gracefully in the event
loop to check should_exit() immediately instead of waiting for timeout.

src/lock.rs

@@ -73,9 +73,11 @@ pub fn create_lock() -> Result<LockGuard> {
     let path = lock_file_path();
 
     // Install SIGUSR1 handler before writing the lock file.
+    // No SA_RESTART — let the signal interrupt poll() so the event loop
+    // notices should_exit() promptly instead of waiting for the timeout.
     let action = SigAction::new(
         SigHandler::Handler(handle_sigusr1),
-        SaFlags::SA_RESTART,
+        SaFlags::empty(),
         SigSet::empty(),
     );
     unsafe { signal::sigaction(Signal::SIGUSR1, &action) }.context("install SIGUSR1 handler")?;