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.md

@@ -20,4 +20,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | window | 19 | 0 | 19 |
 | nettax | 11 | 0 | 11 |
 | stock | 19 | 0 | 19 |
-| **Total** | **258** | **0** | **258** |
+| refund | 20 | 0 | 20 |
+| **Total** | **278** | **0** | **278** |