v2.0.1: G.4 amd64 RDRAND backend behind rng_get_bytes() (SER5 entropy)

First real per-arch RNG backend, added to the v2.0.0 unified entry point in
src/starkernel/rng/rng.c, #if-guarded to amd64: CPUID.01H:ECX[30] RDRAND
detection + inline-asm rdrand draws feeding rdrand_fill() (whole-byte
emission from the low end; a partial final draw is discarded -- throwing
away entropy is always safe).

Probe order honors the release policy: virtio-rng is tried first, so the
QEMU path stays on virtio-rng unchanged; RDRAND is the fallback only real
hardware (which has no virtio-rng device) reaches. QEMU-verified both ways
on amd64: with virtio-rng present -> "rng: backend = virtio-rng" (unchanged);
with virtio-rng absent and RDRAND exposed (-cpu max) -> "rng: backend =
rdrand" + "entropy: ready" + Zuse attach confirmed. rdrand_fill()'s exact
logic host-proven: fills 32-byte/16-byte buffers and yields differing draws
run-to-run (non-deterministic). aarch64/riscv64 builds unaffected (guarded
off). riscv64 Zkr and aarch64 peripheral-RNG backends remain parked for their
real boards.

FABRIC-3.md G.4 amd64 slice marked BUILT + QEMU-verified.

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