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

apps/dhcp/dhcp.c

@@ -0,0 +1,28 @@
+/**
+ * @file dhcp.c
+ * @brief Display DHCP status information.
+ *
+ * Reads DHCP lease information from /sys/dhcp/status and displays it.
+ *
+ * Usage:
+ *   dhcp       - Show current DHCP status
+ */
+
+#include "syscalls.h"
+
+int main(void) {
+    int32_t fd = open("/sys/dhcp/status", 0);
+    if (fd < 0) {
+        puts("dhcp: failed to open /sys/dhcp/status\n");
+        return 1;
+    }
+
+    char buf[512];
+    int32_t n;
+    while ((n = read(fd, buf, sizeof(buf))) > 0) {
+        write(1, buf, (uint32_t)n);
+    }
+
+    close(fd);
+    return 0;
+}