Implement DHCP client subsystem and dhcp app (AI)

- Created src/dhcp.h: DHCP packet struct, lease info struct, message types,
  options codes, client states, discover/receive/get_lease API
- Created src/dhcp.c: DHCP client with DISCOVER/OFFER/REQUEST/ACK flow,
  manual IP+UDP header construction for broadcast, option parsing for
  subnet mask/router/DNS/lease time/server ID, lease table, auto-applies
  configuration to ethernet interface on ACK, sysfs /sys/dhcp/status
- Created apps/dhcp/dhcp.c: reads /sys/dhcp/status to display DHCP info
- Kernel calls dhcp_init() at boot
- Tested: clean boot, DHCP initialized, dhcp app in CPIO

build.log

@@ -6,6 +6,8 @@ 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: dhcp
+  Built: /workspaces/claude-os/build/apps_bin/dhcp (219 bytes)
 Building app: diskpart
 /usr/bin/ld: warning: /workspaces/claude-os/build/apps_bin/diskpart.elf has a LOAD segment with RWX permissions
   Built: /workspaces/claude-os/build/apps_bin/diskpart (8406 bytes)
@@ -37,10 +39,10 @@ Building app: sh
   Built: /workspaces/claude-os/build/apps_bin/sh (3428 bytes)
 [  2%] Built target apps
 [  5%] Generating CPIO initial ramdisk
-Generated initrd: 24432 bytes
+Generated initrd: 24768 bytes
 [  5%] Built target initrd
-[  8%] Building C object src/CMakeFiles/kernel.dir/kernel.c.o
-[ 10%] Building C object src/CMakeFiles/kernel.dir/arp.c.o
+[  7%] Building C object src/CMakeFiles/kernel.dir/kernel.c.o
+[ 10%] Building C object src/CMakeFiles/kernel.dir/dhcp.c.o
 [ 13%] Linking C executable ../bin/kernel
 [ 97%] Built target kernel
 [100%] Generating bootable ISO image
@@ -50,14 +52,14 @@ 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.OdkOfh'
+Added to ISO image: directory '/'='/tmp/grub.dJBlkG'
 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 :  63.88% done
-ISO image produced: 5986 sectors
-Written to medium : 5986 sectors at LBA 0
+xorriso : UPDATE : Thank you for being patient. Working since 0 seconds.
+ISO image produced: 6009 sectors
+Written to medium : 6009 sectors at LBA 0
 Writing to 'stdio:/workspaces/claude-os/release/claude-os.iso' completed successfully.
 
 [100%] Built target iso