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.json

@@ -3,9 +3,10 @@
     "repo": {"pass": 91, "fail": 0},
     "access": {"pass": 103, "fail": 0},
     "wire": {"pass": 78, "fail": 0},
-    "issues": {"pass": 88, "fail": 0}
+    "issues": {"pass": 88, "fail": 0},
+    "pr": {"pass": 100, "fail": 0}
   },
-  "total_pass": 360,
+  "total_pass": 460,
   "total_fail": 0,
-  "total": 360
+  "total": 460
 }