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

build.log

@@ -2,6 +2,8 @@
 -- Generating done (0.2s)
 -- Build files have been written to: /workspaces/claude-os/build
 [  2%] Building user-mode applications
+Building app: arp
+  Built: /workspaces/claude-os/build/apps_bin/arp (214 bytes)
 Building app: cat
   Built: /workspaces/claude-os/build/apps_bin/cat (310 bytes)
 Building app: diskpart
@@ -35,10 +37,10 @@ Building app: sh
   Built: /workspaces/claude-os/build/apps_bin/sh (3428 bytes)
 [  2%] Built target apps
 [  5%] Generating CPIO initial ramdisk
-Generated initrd: 24100 bytes
+Generated initrd: 24432 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
+[ 10%] Building C object src/CMakeFiles/kernel.dir/arp.c.o
 [ 13%] Linking C executable ../bin/kernel
 [ 97%] Built target kernel
 [100%] Generating bootable ISO image
@@ -48,14 +50,14 @@ 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.JjepBC'
+Added to ISO image: directory '/'='/tmp/grub.OdkOfh'
 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 :  0.27% done
-ISO image produced: 5974 sectors
-Written to medium : 5974 sectors at LBA 0
+xorriso : UPDATE :  63.88% done
+ISO image produced: 5986 sectors
+Written to medium : 5986 sectors at LBA 0
 Writing to 'stdio:/workspaces/claude-os/release/claude-os.iso' completed successfully.
 
 [100%] Built target iso