Implement Ethernet subsystem with unified NIC abstraction (AI)

- Created src/ethernet.h: eth_iface_t interface struct, Ethernet header
  type, htons/ntohs/htonl/ntohl byte-order helpers, EtherType constants,
  send/recv with automatic header construction/stripping
- Created src/ethernet.c: interface table, char device ops routed
  through ethernet layer, sysfs /sys/net namespace exposing per-iface
  mac/ip/netmask/gateway/link files, IPv4 address parse/format
- NE2000 and 3C509B drivers now register through ethernet_register()
  instead of directly via devicefs_register_char(); removed redundant
  char device ops from both drivers
- Kernel calls ethernet_init() before init_drivers() so the subsystem
  is ready when NIC drivers probe
- Tested: NE2000 detected with NIC, 'eth1' registered via ethernet
  subsystem; clean boot without NIC

src/kernel.c

@@ -22,6 +22,7 @@
 #include "mbr.h"
 #include "fat32.h"
 #include "keyboard.h"
+#include "ethernet.h"
 #include "framebuffer.h"
 
 /* Global framebuffer info, parsed from multiboot2 tags. */
@@ -415,6 +416,9 @@ void kernel_main(uint32_t magic, uint32_t addr) {
         fb_info.pitch = 80 * 2;
     }
 
+    ethernet_init();
+    offset_print("Ethernet subsystem initialized\n");
+
     init_drivers();
     EARLY_PRINT("DRV ");
     offset_print("Drivers initialized\n");