nvx-rs: replace activation RPC delivery with an uncaptured VMM configuration region #544

Parent: #535. Phase 8 (delivery refinement). Depends on the functional authenticated RPC path #542 and full mapped/networked template proof #543. Follow the proposed replaceable configuration region, but do not substitute region design work for a working restore. This issue also tracks the separate OpenVMM implementation and exact source pin: the existing command-line token alone does not allocate a region, exclude it from captured RAM or implement the payload contract.

Goal

Replace only the small activation payload's delivery with a bounded, versioned region populated by OpenVMM outside captured RAM on cold launch and platform restore. Keep the same validated semantic configuration, authenticated host/guest identities, runtime transports and bidirectional credit-based exec behavior proven in earlier phases. Do not put tenant settings, mapped host paths, or an active control session into the reusable snapshot.

Contract

• Define immutable platform/header invariants and independently replaceable image-binding and per-sandbox sections: versions, lengths, generation/config identity, expected HostMap count (including zero), selected image role (distro for direct-image cold boot or the sole image0 for a template), guest network/DNS and proxy. The region cannot select another slot or change the captured slot topology. OpenVMM allocates and maps the bounded region outside captured RAM (including guest memory-map and memory.bin exclusion), fills/validates per-launch sections before vCPU resume and versions the template machine contract. Put the new region ABI in the immutable template compatibility key.
• The host authenticates and validates the selected configuration, and OpenVMM validates and installs its region sections plus the VMM network policy before any restored vCPU executes. Only after resume can the guest check version, bounds, invariants and accepted identity before mounts/network/Ready; the host checks its echo before reporting Ready/allowing exec. Do not claim the guest consumed a section before any vCPU ran. A checksum detects corruption but is not authorization: preserve authenticated host control and enforce the VMM policy independently of guest cooperation.
• Continue delivering nonempty potentially large HostMap tables in bounded ordered batches on the authenticated control channel; the region carries their expected count, not host-file contents. Public NVXC and other adapters keep their own application framing.
• Retire the interim activation RPC only when both cold and restored new-profile launches use the region, have identical observable behavior, and report Ready with the selected identity. Unsupported region/version, stale generation, changed image/policy or partial application fails closed; explicitly supported old NVXC v1 bundles keep their pinned contract, with no silent downgrade.
• Keep the region inaccessible to workloads. No device hotplug and no workload-after-exec snapshots are added.

Acceptance

• Cold and restored boots with zero/one/many mappings and different image/network/proxy policies have the same outcomes as the prior RPC profile.
• Tests cover bounds, unknown versions, truncated/mismatched sections, replayed identity/generation, an invalid image role or old four-slot contract, corruption, authentication failure, inaccessible-to-workload property and VMM enforcement before vCPU resume.
• A real restored VM proves tenant settings and region pages do not reside in captured RAM/memory.bin, guest bounds and workload isolation hold, and an old snapshot does not silently accept the new region contract. Record and test the reviewed OpenVMM PR/source pin separately from the NVX adapter.
• Compare source-matched RPC and region boot-to-Ready timings with identical resources; no latency improvement is claimed from a skipped map batch, guest step or policy check.