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
d7d7e8e58e