Design: representation of Update auxiliary fields

[wjones127]

Parent PRD

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

What to decide

Operation::Update carries several auxiliary fields that do not map onto any action type defined in milestone 11 (#6450–#6453):

• fields_modified
• merged_generations
• fields_for_preserving_frag_bitmap
• update_mode
• inserted_rows_filter
• updated_fragment_offsets

These are mostly stats/flags consumed during apply or by downstream code paths (e.g. index maintenance, MemWAL), not state mutations of the manifest. They block lossless translation of Operation::Update to actions.

Options

1. Ride-along on UserAction — extend UserAction (or UserOperation) with an optional metadata payload for these fields. They participate in serialization but not in apply or conflict detection. Cheapest; preserves round-trip.
2. First-class actions — define one action per field (e.g. RecordMergedGenerations, SetUpdateMode). Heaviest; uniform but most ceremony.
3. Audit and drop — determine which fields are actually read downstream and deprecate the rest. Likely correct for at least some (e.g. inserted_rows_filter, updated_fragment_offsets may be stats only).
4. Hybrid — audit first (option 3); for any survivors, ride-along (option 1).

Recommended starting point: option 4. Audit fields_modified, merged_generations, update_mode, inserted_rows_filter, updated_fragment_offsets, fields_for_preserving_frag_bitmap for actual consumers, then decide per-field.

Acceptance criteria

• Each field documented with its current consumer(s) in the codebase
• Per-field decision recorded: drop / ride-along / first-class action
• Design written up here or in a short ADR
• Unblocks the Update Operation translation (separate follow-up implementation issue)

Blocked by

• Blocked by RemoveFragments + UpdateFragment actions (internal) #6450 (action vocabulary baseline)

User stories addressed

• User story 9: old operations translated to actions
• User story 11: lossless round-trip