nvx-rs: NVX's platform-clone scenarios and documentation #854

Parent: #837 (#833). Needs: #853. Unblocks: #855.

Goal. NVX's adapter runs its platform-clone scenarios in CI on every backend, and its documentation describes the commands and the guest's behavior.

Today (dev d04d9fb): NVX's microVM suite drives platform snapshots from a script (scripts/nvx_tools/microvm_tests.py:7008-7011, :7254-7320), outside nvx::host.

Implementation:

1. Scenarios, in NVX's microVM suite through nvx-rs sandbox, on KVM, MSHV and WHP in CI:
   • EROFS and GPT, over tmpfs and scratch, from one template;
   • shorter images; CPU and memory targets; per-clone network policies; aggregate exports;
   • each failure status; reattach after a host restart.
2. Sizes: the guest binaries' and initramfs's sizes, before and after this workstream, in the suite's report.
3. Documentation: the CLI's reference for the template and clone commands, and the guide's NVX chapters (doc: describe the managed layered root and platform-clone activation #591).

Done when: the scenarios pass in CI on the three backends, and the documentation is merged.