Add initial virtio-net foundation

[elegantmonark]

Summary

Hi Troy, this PR adds the first virtio-net foundation for Caffeinix.

It introduces shared virtio MMIO/virtqueue definitions, wires a second virtio-mmio
device into QEMU, adds VIRTIO1 memory/IRQ constants, initializes a virtio-net device,
prepares RX/TX virtqueues, and routes virtio-net interrupts through the PLIC/trap
path.

Scope

This intentionally stops at device bring-up and queue initialization.

It does not add a full networking stack, packet RX/TX APIs, ARP, IP, TCP/UDP,
sockets, DHCP, or userspace networking yet.

Why

Caffeinix already has virtio block support, so virtio-net is a natural next step
toward OS-level networking support under QEMU/RISC-V.

Testing

• Ran git diff --check
• Local build not run because this Windows environment does not have the Caffeinix
  RISC-V toolchain/QEMU or Docker installed
• Existing GitHub Actions should build this through troymitchell/caffeinix:1.0