Schema actions: AddFields, DropFields, UpdateSchemaMetadata (internal)

[wjones127]

Parent PRD

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

What to build

Add schema action types through all internal layers (types, translation, apply, conflict detection).

• AddFields: Add new fields to the schema. Covers AddColumns, Merge (schema part), adding fields during Overwrite.
• DropFields: Drop fields from the schema by field ID. Covers DropColumns (schema-level; data tombstoned via UpdateFragment).
• UpdateSchemaMetadata: Update schema-level metadata without changing fields.

Operation translations:

• Merge → [AddFields + AddFragments/UpdateFragment]
• Project → [DropFields]

Each translation must round-trip losslessly.

Acceptance criteria

• AddFields, DropFields, UpdateSchemaMetadata types defined and apply logic implemented
• Conflict detection for schema actions
• Operation translations implemented with round-trip tests
• Existing merge/project tests still pass

Blocked by

• Blocked by Tracer bullet: AddFragments action end-to-end (internal) #6449 (tracer bullet — need the framework)

User stories addressed

• User story 5: single code path for applying operations
• User story 9: old operations translated to actions
• User story 11: lossless round-trip