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

src/kernel.c

@@ -24,6 +24,7 @@
 #include "keyboard.h"
 #include "ethernet.h"
 #include "ipv4.h"
+#include "arp.h"
 #include "framebuffer.h"
 
 /* Global framebuffer info, parsed from multiboot2 tags. */
@@ -423,6 +424,9 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     ipv4_init();
     offset_print("IPv4 stack initialized\n");
 
+    arp_init();
+    offset_print("ARP subsystem initialized\n");
+
     init_drivers();
     EARLY_PRINT("DRV ");
     offset_print("Drivers initialized\n");