Define generation IDs, restore entropy, and VP targets

Tracking issue: MicroVM support and private snapshot restore.

Motivation

Cloning a prepared guest must not silently reuse state that is intended to be
instance-specific. Generation changes and fresh entropy need a guest-visible
contract, and requested VP targets must agree with the saved topology. The host
also needs to know when guest-side repair is complete before exposing the instance
as ready for work.

Proposed Scope

• Define generation-ID handling for restored instances and make the intended
  transition observable to the guest.
• Provide gated restore-time entropy using an appropriate host source, only when
  the guest contract supports consuming it safely.
• Validate requested VP restore targets against the supported topology, capacity,
  and processor-state contract.
• Add a guest repair/readiness protocol that orders identity refresh, entropy
  consumption, and VP preparation before workload readiness.
• Bound waiting and propagate unsupported capabilities or repair failures through
  the restore transaction.

Acceptance Criteria

• Independent clone restores observe distinct generation identities under the
  documented policy.
• Controlled tests verify entropy delivery, capability gating, and host entropy
  failure without relying on probabilistic uniqueness assertions.
• Unsupported or out-of-range VP targets are rejected before affected VPs run.
• A source-built guest demonstrates the repair/readiness handshake and does
  not report workload readiness before required repair completes.
• Missing acknowledgment, cancellation, and repair failure produce bounded
  outcomes with clean teardown.
• Documentation states guest responsibilities and makes clear that supplying
  entropy alone does not repair every guest-cached identity or random state.

Dependencies

• E02: Dormant filesystem slots and restore-time attachment
  establishes the restore-time preparation sequence to extend.
• E01: Canonical topology supplies VP identity and
  topology validation.

Non-Goals

This issue does not promise transparent cloning of unmodified guests or arbitrary
CPU hotplug. Snapshot capture tiers and block-role policy are defined separately.

Start with the tracking issue in 00-microvm.md, then read E02 (#4443) for the restore-time preparation sequence and E01 (#4442) for VP identity and topology validation. Map the generation, entropy, readiness, timeout, and failure requirements to the existing restore flow. Done means the acceptance criteria are met, including controlled tests and documentation of guest responsibilities.