Implement ARP subsystem and arp app (AI)

- Created src/arp.h: ARP packet struct, cache entry struct, operation codes,
  lookup/request/resolve/receive API, sysfs registration
- Created src/arp.c: ARP cache with 32 entries, request/reply handling,
  ARP response to incoming requests for our IP, sysfs /sys/arp/table
  with formatted IP/MAC/interface/state columns
- Created apps/arp/arp.c: reads and displays /sys/arp/table
- Kernel calls arp_init() at boot, registered sysfs 'arp' namespace
- Tested: clean boot, ARP initialized, arp app in CPIO

apps/arp/arp.c

@@ -0,0 +1,30 @@
+/**
+ * @file arp.c
+ * @brief Display the ARP table.
+ *
+ * Reads the ARP cache from /sys/arp/table and displays it.
+ *
+ * Usage:
+ *   arp       - Show the ARP table
+ */
+
+#include "syscalls.h"
+
+int main(void) {
+    /* Open the ARP table sysfs file */
+    int32_t fd = open("/sys/arp/table", 0);
+    if (fd < 0) {
+        puts("arp: failed to open /sys/arp/table\n");
+        return 1;
+    }
+
+    /* Read and display contents */
+    char buf[512];
+    int32_t n;
+    while ((n = read(fd, buf, sizeof(buf))) > 0) {
+        write(1, buf, (uint32_t)n);
+    }
+
+    close(fd);
+    return 0;
+}