Implement IPv4 stack and ip app (AI)
- Created src/ipv4.h: IPv4 header struct, protocol numbers, checksum,
address conversion (ipv4_aton/ntoa), send/receive/routing API,
protocol handler registration
- Created src/ipv4.c: packet construction with header checksum,
simple routing (direct subnet + gateway), incoming packet validation
and dispatch to registered protocol handlers
- Created apps/ip/ip.c: displays network interface config from
/sys/net (MAC, link, IP, netmask, gateway); supports 'ip set'
to configure interface via sysfs writes
- Added ipv4.c to kernel build, kernel calls ipv4_init() at boot
- Tested: clean boot, IPv4 initialized, ip app in CPIO
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