Add initial layered IPv4 networking stack (Ethernet/ARP/IPv4/ICMP/UDP/DNS + ping/wget scaffolding)

[pradosh-arduino]

Motivation

• Provide a small, maintainable Linux-like layered networking stack on top of the existing RTL8139 driver so upper layers never touch hardware registers.
• Ship core link/network/transport primitives (Ethernet, ARP, IPv4, ICMP, UDP, DNS) and a simple application surface so the OS can ping and attempt HTTP downloads.
• Keep the driver integration minimal and clear by exposing a netif_* boundary that hides rtl8139 details from protocol code.

Description

• Add a public networking header and packet buffer abstraction via source/includes/net/net.h and net_packet_t to allow safe prepend/append/pull operations and headroom management.
• Implement core modules: net_core.c (byte-order, checksum, packet buffer, netif_* wrapper), ethernet.c (Ethernet II parsing/construction and EtherType dispatch), and arp_ipv4_icmp_udp.c (ARP cache & resolution, IPv4 parsing/building/checksum, ICMP echo, UDP send/receive queues).
• Add dns_tcp_http_dhcp.c with a UDP DNS resolver and an HTTP client that is wired to the networking API while TCP is provided as a scaffold (TCP state machine/retransmit/congestion features are noted as pending full wire-mode implementation); DHCP is a placeholder.
• Integrate with shell by adding ping and wget commands, register them in the shell table, and initialize the stack during startup with QEMU user-network defaults via ipv4_init(...); document architecture and QEMU testing notes in source/kernel/C/net/README.md.

Testing

• Ran make -C source which compiled the new networking C files and integrated objects into the build, but the final link failed due to pre-existing build environment issues where versions.sh could not generate versions/date symbols (xorriso missing), so the linker reported unrelated undefined references; all new networking objects compiled successfully before the link step.
• The build run demonstrated that the new files are included in the build system and that netif_send/netif_poll are wired to the existing rtl8139_send_packet/rtl8139_receive_packet polling path as intended.
• No further automated network runtime tests were executed in this environment; recommended runtime validation is described in the new README (example: QEMU with -netdev user,id=n0 -device rtl8139,netdev=n0 and ping 10.0.2.2 / wget http://...).

---

Codex Task