PMC vote: new protobuf format for Action-based transactions

Parent PRD

Milestone: Action-based Transactions (UserOperation) — see milestone description for full PRD.
Discussion: #5960

What to build

Submit the new protobuf message definitions for PMC vote (1-week voting period). Messages include:

• UserOperation (uuid, read_version, repeated UserAction)
• UserAction (description, repeated Action)
• Action oneof with all 14 variants
• All individual action messages (AddFragments, RemoveFragments, UpdateFragment, AddFields, DropFields, UpdateSchemaMetadata, AddIndex, RemoveIndex, UpdateConfig, UpdateTableMetadata, SetDataFormat, SetFeatureFlags, ReserveFragmentIds, UpdateMemWalIndex)
• Top-level Transaction oneof: UserOperation | Restore | Clone

The proto definitions should be informed by the proven internal architecture from the tracer bullet.

Acceptance criteria

• Proto definitions drafted and submitted for PMC vote
• 1-week voting period completed
• Vote passed

Blocked by

• Blocked by #6449 (tracer bullet — need proven architecture before voting)

User stories addressed

• User story 8: feature flag compatibility
• User story 12: writer feature flag for cutover control

Start with the Action-based Transactions milestone description and discussion #5960, then review the architecture produced by blocked issue #6449. Draft the listed transaction and action messages for PMC submission; done requires the definitions to be submitted, the one-week vote to complete, and the vote to pass.