sx-gitea Phase 5: pr — merge-base diffs, reviews, flow lifecycle, 3-way merge, merge queue (TDD, 460/460)

lib/gitea/pr.sx: PRs as kv records sharing the per-repo number counter
with issues. Diffs are LIVE, computed from the merge base of the current
branch heads to the source head via sx-git (no spurious deletions when
the target moves on). Reviews: latest verdict per reviewer wins; authors
cannot review their own PR; approved? = some approve and no outstanding
request-changes.

Lifecycle is a lib/flow durable workflow (deterministic-replay suspend):
open -(approval)-> approved -(merge)-> merged; review! resumes the
approval suspend when the verdict set first approves, merge! resumes the
rest, close! cancels, reopen! starts a fresh flow. The flow env lives in
the forge handle; the record's :state stays the source of truth.

Merge via git/merge-commits over the merge base: up-to-date, fast-
forward (ref move only), true two-parent merge commit, or conflicts with
the conflicting paths. Every ref move is branch-cas! — concurrent pushes
surface as 'stale'. Merge queue: approved PRs merge in order,
failures stay queued.

Web: pulls list + PR page (body html, reviews, lifecycle, unified diff),
JSON API for create/review/merge (409 on conflicts/stale)/close (author
or write)/enqueue/queue-process.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/gitea/scoreboard.md

@@ -8,4 +8,5 @@ _Generated by `lib/gitea/conformance.sh`_
 | access | 103 | 0 | 103 |
 | wire | 78 | 0 | 78 |
 | issues | 88 | 0 | 88 |
-| **Total** | **360** | **0** | **360** |
+| pr | 100 | 0 | 100 |
+| **Total** | **460** | **0** | **460** |