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

build.log

@@ -1,3 +1,6 @@
+-- Configuring done (0.1s)
+-- Generating done (0.2s)
+-- Build files have been written to: /workspaces/claude-os/build
 [  2%] Building user-mode applications
 Building app: cat
   Built: /workspaces/claude-os/build/apps_bin/cat (310 bytes)
@@ -11,6 +14,8 @@ Building app: fork-test
   Built: /workspaces/claude-os/build/apps_bin/fork-test (132 bytes)
 Building app: hello-world
   Built: /workspaces/claude-os/build/apps_bin/hello-world (49 bytes)
+Building app: ip
+  Built: /workspaces/claude-os/build/apps_bin/ip (3695 bytes)
 Building app: ls
   Built: /workspaces/claude-os/build/apps_bin/ls (250 bytes)
 Building app: mkfs.fat32
@@ -29,7 +34,12 @@ Building app: sh
 1 warning generated.
   Built: /workspaces/claude-os/build/apps_bin/sh (3428 bytes)
 [  2%] Built target apps
+[  5%] Generating CPIO initial ramdisk
+Generated initrd: 24100 bytes
 [  5%] Built target initrd
+[  8%] Building C object src/CMakeFiles/kernel.dir/kernel.c.o
+[ 11%] Building C object src/CMakeFiles/kernel.dir/ipv4.c.o
+[ 13%] Linking C executable ../bin/kernel
 [ 97%] Built target kernel
 [100%] Generating bootable ISO image
 xorriso 1.5.6 : RockRidge filesystem manipulator, libburnia project.
@@ -38,14 +48,14 @@ Drive current: -outdev 'stdio:/workspaces/claude-os/release/claude-os.iso'
 Media current: stdio file, overwriteable
 Media status : is blank
 Media summary: 0 sessions, 0 data blocks, 0 data,  126g free
-Added to ISO image: directory '/'='/tmp/grub.KhfgHk'
+Added to ISO image: directory '/'='/tmp/grub.JjepBC'
 xorriso : UPDATE :     581 files added in 1 seconds
 Added to ISO image: directory '/'='/workspaces/claude-os/build/isodir'
 xorriso : UPDATE :     586 files added in 1 seconds
 xorriso : NOTE : Copying to System Area: 512 bytes from file '/usr/lib/grub/i386-pc/boot_hybrid.img'
-xorriso : UPDATE : Thank you for being patient. Working since 0 seconds.
-ISO image produced: 5967 sectors
-Written to medium : 5967 sectors at LBA 0
+xorriso : UPDATE :  0.27% done
+ISO image produced: 5974 sectors
+Written to medium : 5974 sectors at LBA 0
 Writing to 'stdio:/workspaces/claude-os/release/claude-os.iso' completed successfully.
 
 [100%] Built target iso