Add psignal module for centralized signal handling

- Added pclsync/psignal.h with psignal_register() and psignal_check_pending() API
- Added pclsync/psignal.c with async-signal-safe handlers for SIGINT, SIGTERM, SIGHUP
- Implements flag-based deferred signal handling pattern

pclsync/psignal.c

@@ -0,0 +1,61 @@
+#include "psignal.h"
+#include <signal.h>
+#include <stddef.h>
+
+static volatile sig_atomic_t sigint_flag = 0;
+static volatile sig_atomic_t sigterm_flag = 0;
+static volatile sig_atomic_t sighup_flag = 0;
+
+static void sigint_handler(int sig) {
+  sigint_flag = 1;
+}
+
+static void sigterm_handler(int sig) {
+  sigterm_flag = 1;
+}
+
+static void sighup_handler(int sig) {
+  sighup_flag = 1;
+}
+
+void psignal_register(int signum) {
+  struct sigaction sa;
+  void (*handler)(int) = NULL;
+  
+  if (signum == SIGINT) {
+    handler = sigint_handler;
+  } else if (signum == SIGTERM) {
+    handler = sigterm_handler;
+  } else if (signum == SIGHUP) {
+    handler = sighup_handler;
+  } else {
+    return;
+  }
+  
+  sa.sa_handler = handler;
+  sigemptyset(&sa.sa_mask);
+  sa.sa_flags = 0;
+  if (sigaction(signum, &sa, NULL) != 0) {
+    return;
+  }
+}
+
+int psignal_check_pending(void) {
+  sig_atomic_t int_flag = sigint_flag;
+  sig_atomic_t term_flag = sigterm_flag;
+  sig_atomic_t hup_flag = sighup_flag;
+  
+  if (int_flag) {
+    sigint_flag = 0;
+    return SIGINT;
+  }
+  if (term_flag) {
+    sigterm_flag = 0;
+    return SIGTERM;
+  }
+  if (hup_flag) {
+    sighup_flag = 0;
+    return SIGHUP;
+  }
+  return 0;
+}

pclsync/psignal.h

@@ -0,0 +1,9 @@
+#ifndef PSIGNAL_H
+#define PSIGNAL_H
+
+#include <signal.h>
+
+void psignal_register(int signum);
+int psignal_check_pending(void);
+
+#endif