Four capabilities that were already on the wire but never decoded:
draft, review_requests, stacked_on and stacked are all omitempty
on mr show and mr list, so absence is the ordinary case and the
models default rather than require.
Draft is a flag, not a fifth state — state stays open, and a draft
simply does not merge, which the merge control now says rather than
offering a button the server refuses. mr create gains a Draft toggle.
Retarget picks a branch from repo refs and warns that approvals go
stale, since an approval was of the diff against the old target. The
picker distinguishes loading, failed and genuinely-empty rather than
showing an empty list for all three.
The stack reads in both directions — what this is stacked on, and what is stacked on it — each linking through. Squash and rebase are hidden while anything is stacked, since they would rewrite the commits the stack builds on; fast-forward and a merge commit both remain, which is what the server actually permits.
Reviewers can be requested and removed through mr review request.
Closes four parity rows: draft/ready, retarget, request a review, and stacked merge requests.