state: Couple drafts to branch lifecycle

[abhinav]

Review drafts live separately from branch records.
Their ID allocator therefore persists after publication.
Deleting or renaming a branch without updating that document
leaves orphaned state or strands drafts under an obsolete name.

Delete or move the draft document with the branch state transaction.
Draft mutations read the owning branch from the same snapshot.
A concurrent untrack therefore cannot recreate discarded state.

[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.}