Phase 8 A: virtio-rng entropy source for real Ed25519 signing

The kernel's ed25519_verify() is deliberately verify-only -- no signing,
no keygen, no entropy source. That conflicts with the on-device MINT
word vision (Zuse signing new user certs live at runtime), so this
reopens that constraint on request rather than reshaping MINT around
verify-only.

vm_uuid.h already found the real gap: amd64 has RDRAND, riscv64 has Zkr,
but QEMU's aarch64 CPU models have neither -- confirmed against QEMU
10.2.1. A deterministic PRNG (fine for VM UUIDs) is not safe for key
generation, so this adds a virtio-rng device instead of a per-arch split:
real host entropy, identical guest-side protocol on all three arches.

New src/starkernel/virtio/virtio_rng.c + include/starkernel/virtio_rng.h,
transport plumbing mirroring the existing virtio_blk.c driver exactly.
Wired into kernel_main.c boot, -device virtio-rng-pci added to all three
QEMU targets.

Verified live (temp probe, written/run/captured/reverted): 16 real bytes
pulled through the full request/notify/poll round trip on all three
arches, three different values confirming real entropy. Final boot
against the reverted, permanent code: clean compile, clean boot to ok>
on amd64/aarch64/riscv64, Stadium conservation intact, no panics or
guest errors.

Ed25519 keygen/signing itself (Phase B) and the MINT word design
(Phase C) remain open, documented in FABRIC-3.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U14ET9CWAtbQMbYqomKgXd

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