Support cloning type-erased protobuf values

Tracking issue: MicroVM support and private snapshot restore.

Motivation

Transactional lifecycle operations may need to retain a saved value while another
operation consumes an equivalent value. Type-erased protobuf storage should expose
clear ownership and clone semantics so callers do not need to recover the concrete
message type or invent an application-specific serialization workaround.

Proposed Scope

• Provide clone support for the relevant type-erased protobuf value abstraction.
• Define how cloning preserves the payload and its ownership semantics, including
  any required type bounds or restrictions.
• Preserve protobuf encoding compatibility and existing conversion behavior.
• Keep lifecycle policy outside the serialization abstraction.

Acceptance Criteria

• An erased value and its clone encode equivalent protobuf payloads and
  successfully round-trip through the supported decode path.
• The clone remains usable after the original value is consumed or dropped.
• Tests cover representative owned payloads, including nested messages and
  variable-length data, without accidental shared mutable state.
• The API documents its clone semantics and applicable ownership constraints.
• Existing serialization tests continue to pass without a wire-format change.

Dependencies

No feature prerequisites. Fallible lifecycle transactions will consume this
capability in a separate PR.

Non-Goals

This issue does not define snapshot manifests, lifecycle rollback, or device
persistence policy.