nvx-rs: capture preparation in the guest, and template mode in the test adapter's guest #840

Parent: #834 (#833), steps 1 and 2. Needs: nothing; its VM tests run with #841. Unblocks: #841, #842.

Goal. A template's guest captures before root assembly, through a typed preparation in nvx::guest that a privileged guest agent can call without being PID 1. The test adapter's guest gets a template mode that uses it.

Today (dev d04d9fb):

• The shell nvx-snapshot checks the host-configured tier, reads the generation ID, runs nvx-time pre-capture, freezes the workload cgroup (and the scratch, when paired), then runs nvx-time capture. A successful capture ends the source VM (guest/common/nvx-snapshot:256-280, :448-497).
• nvx::guest::time::TimeAbi::capture(CaptureRequest) returns CaptureOutcome::Continued or CaptureOutcome::Restored(Restore) (nvx-rs/src/guest/time/api.rs:96-160, :264-355). time::system saves and restores the stall detectors' settings: RCU stalls, soft lockups and hung tasks (nvx-rs/src/guest/time/system.rs:104-108, :473-507).
• NVX's layered init assembles the root first, so no capture can come before root assembly. The test adapter's guest runs a PID 1 that supervises a worker, which assembles the root when the host sends RootConfig (tests/runtime-adapter/src/bin/runtime-adapter-guest.rs, src/worker.rs:518-601, src/frame.rs).

Stacked PRs. PR 2 builds on PR 1; each lands with its own tests.

PR 1 of 2: capture preparation in nvx::guest. Step 1; needs nothing.

• A new module, nvx::guest::clone, with CapturePreparation. The caller supplies its workload-admission fence and the device identities of distro, the scratch and the filesystem tag.
• CapturePreparation::prepare returns an owned CaptureBarrier after it:
  • fences workload admission through the caller's fence, so that no execution starts;
  • checks, with a bounded read of /proc/self/mountinfo, that nothing is mounted from distro, the overlay, the scratch or the filesystem tag;
  • waits for the time ABI's boot checks, runs the capture checks, and suppresses the stall detectors through time::system.
• CaptureBarrier::capture calls TimeAbi::capture with the platform tier's request. It returns Captured::Continued, for a source that keeps running, with the barriers released, or Captured::Restored, which hands the restore result to repair (nvx-rs: repair a restored clone in the guest, with an owned seed and one acknowledgment #842), with the detectors still suppressed. For a platform template, OpenVMM ends the source VM after the capture, so only clones return from it.
• CaptureBarrier::cancel, and dropping the barrier, release the fence and restore the detectors.
• A typed CaptureError, with the stage and the cause.
• Tests, with fakes for mountinfo, the fence and the time ABI: the order (the fence before the checks, and the detectors suppressed before the capture); each mount that blocks a capture; cancel and drop restore the detectors and release the fence.

PR 2 of 2: template mode in the test adapter's guest. Step 2; needs PR 1.

• The guest reads /proc/cmdline early, through nvx::protocol::cmdline::parameters. With nvx_snapshot_tier=platform, it accepts only the platform allowlist's tokens, and refuses any other.
• It skips root assembly, attaches the control session with nvx::guest::nvxs::GuestConnection::attach, and runs the preparation and the capture. No application session runs before the capture.
• A restored guest continues with repair. Until nvx-rs: repair a restored clone in the guest, with an owned seed and one acknowledgment #842 lands, it shuts down with a documented status. CaptureError maps to the guest's exit status.
• Tests: template mode refuses a token outside the allowlist. nvx-rs: lower, capture and publish platform templates #841's captures run this mode in VMs on KVM, MSHV and WHP.

Not in scope: NVX's init (#852); repair (#842); the host's capture (#841).

Guide (#591's numbering): 3.1.3 (the snapshot request), 5.3.2 (guest preparation, before root assembly), 5.3.6 (the guest's barriers), 4.1.4 (template mode skips the managed layered root's assembly).

Done when: both PRs are merged with their tests.