Phase 8 C (1/n): QEMU harness now preserves NVRAM across runs

Scoping Phase C (the MINT word) surfaced a real blocker: "mint one
Zuse, ever" needs the cert to survive reboots, but the qemu target
copied a fresh, pristine OVMF_VARS.fd on every invocation -- a
UEFI-NVRAM-based cert would never persist under this project's own
normal test workflow. Digging further, aarch64 had no persistent NVRAM
store at all (single -bios arg, no split VARS pflash like amd64/riscv64).

Fixed rather than switching storage substrates: amd64/riscv64 now only
copy the VARS template if the destination doesn't already exist, so
`clean` (which deletes the whole build tree) is the bleach step and a
bare `make qemu` preserves NVRAM -- matching the existing "always clean
before qemu" acceptance convention exactly. aarch64 restructured to
split CODE(ro)/VARS(rw) pflash drives matching the other two, with a
graceful fallback to the old -bios mode on hosts without split firmware.

Also resolves two design questions before any cert code: Zuse doesn't
need Milestone 6's CA (that's the capsule-signing chain, a separate
trust domain -- Zuse is a self-sovereign instance-local root of trust),
and flags that this session's own earlier vm_zuse_cert_install() storage
(16 bytes) is too small for a real Ed25519 keypair.

Verified: all three architectures boot clean to ok> with the new pflash
arrangement, Stadium conservation intact, no panics or guest errors.
Infrastructure-only -- no cert code yet. 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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