Define sandbox block roles and clone/resume snapshot tiers

[ppenna]

Tracking issue: MicroVM support and private snapshot restore.

Motivation

A snapshot intended for independent clones has different storage and identity
requirements from one intended to resume an instance. Explicit block roles and
capture tiers would prevent a restore from treating mutable scratch data as a
shareable template or applying clone policy to state that requires resume semantics.

Proposed Scope

• Define supported sandbox block roles and their identity, mutability, ownership,
  and attachment requirements.
• Define explicit clone and resume capture tiers, recording the selected tier in
  the authoritative snapshot contract.
• Specify scratch-state identity and publication together with its clone/resume
  behavior, including which data is retained, reset, or privately reconstructed.
• Validate tier, block roles, attachment identities, and generation policy as one
  coherent capture/restore request.
• Integrate block-state publication and cleanup with the existing transaction.
• Document the role/tier compatibility matrix and unsupported combinations.

Acceptance Criteria

• Tests cover each supported role/tier combination and reject unsupported or
  ambiguous combinations before publication or guest execution.
• Clone restores do not accidentally share mutable scratch state or write into
  immutable template storage.
• Resume restores preserve the storage identity and contents promised by their
  tier and reject incompatible replacement attachments.
• Scratch publication failure leaves no apparently complete snapshot referring
  to incomplete or incorrectly identified block state.
• VMM tests distinguish clone isolation from resume continuity through actual
  guest block I/O.
• CLI/configuration documentation states tier selection and its relationship
  to generation IDs and guest readiness.

Dependencies

• E03: Generation IDs, entropy, and VP restore targets.

Non-Goals

This issue does not introduce general-purpose storage replication or a snapshot
of arbitrary external block services. Authoritative RPC exposure of the tiered
operations is a separate integration PR.