[Black7i77]

Summary

Adds the Linux95 UDP networking milestone on top of the existing
RTL8139 → Ethernet → ARP → IPv4 stack.

Included

• Generic UDP packet build/parse support
• IPv4 UDP checksum handling
• Fixed UDP port bindings
• IPv4 receive dispatch for protocol 17
• UDP transmit path with ARP resolution and bounded pending-send state
• Real QEMU UDP echo verification
• Hardened QEMU smoke-test completion checks
• UDP design spec and implementation plan

Preserved behavior

• Existing RTL8139 networking
• Ethernet / ARP / IPv4 / ICMP ping
• Graphical desktop
• Ring 3 userspace
• Timer-driven preemptive scheduling
• No-network fallback

Verification

• make clean && make all && make test — PASS
• make test-qemu — PASS
• Standalone QEMU modes — UDP echo, no-network, no-user-programs,
  process self-test, process fault, and process preemption — PASS
• nm -u build/kernel.elf — no unresolved symbols
• git diff --check — clean

DNS remains out of scope for this PR and is intended as the next
networking layer.