commerce: refund lifecycle as a flow-on-sx flow (20 tests) — Phase 5 backlog complete

refund.sx — refund as a second flow-on-sx flow (request -> approve -> settle)
with two suspension points (approval = human/policy decision, settle =
provider). refund-begin! records :refund-requested and suspends at approval;
refund-approve! advances to settle; refund-settle! records :refunded
(idempotent) and completes; refund-reject! records :refund-rejected and cancels.
Only :refunded moves the books. Reuses order.sx flow helpers. Completes the
Phase 5 backlog. Total 278/278 across 17 suites.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/commerce/scoreboard.json

@@ -15,9 +15,10 @@
     "payment": {"pass": 7, "fail": 0},
     "window": {"pass": 19, "fail": 0},
     "nettax": {"pass": 11, "fail": 0},
-    "stock": {"pass": 19, "fail": 0}
+    "stock": {"pass": 19, "fail": 0},
+    "refund": {"pass": 20, "fail": 0}
   },
-  "total_pass": 258,
+  "total_pass": 278,
   "total_fail": 0,
-  "total": 258
+  "total": 278
 }