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

src/kernel.c

@@ -25,6 +25,7 @@
 #include "ethernet.h"
 #include "ipv4.h"
 #include "arp.h"
+#include "dhcp.h"
 #include "framebuffer.h"
 
 /* Global framebuffer info, parsed from multiboot2 tags. */
@@ -427,6 +428,9 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     arp_init();
     offset_print("ARP subsystem initialized\n");
 
+    dhcp_init();
+    offset_print("DHCP subsystem initialized\n");
+
     init_drivers();
     EARLY_PRINT("DRV ");
     offset_print("Drivers initialized\n");