state: Make review draft updates atomic

[abhinav]

Concurrent review commands can derive changes from one draft document.
Saving either precomputed document can discard the winning change
or assign one branch-local ID twice.

Store drafts in an ID-keyed object
and update additions and edits with replayable JSON transformations.
Concurrent additions receive distinct IDs,
while edits to separate drafts preserve both changes.

[abhinav]

This change is part of the following stack:

• reviewdiff: Parse review comment patches #1162
  • shamhub: Compute review thread staleness #1222
    • Add review comment commands #1163
      • reviewdiff: Stream branch diffs #1431
        • review: Separate commands from workflows #1432
          • state: Make review draft updates atomic #1438 ◀
            • state: Couple drafts to branch lifecycle #1439
              • state: Preserve drafts changed during publication #1440
                • review: Delete local drafts #1441
                  • review: Configure the draft default #1442
                  • review: Accept file and range drafts #1443

_{Change managed by git-spice.}