commerce: provider-neutral payment-request envelope (8 tests) — Phase 5 ext

payment.sx — payment-request materialises {:order :amount :currency :return-url}
at the IO edge (amount from the ledger, currency/return-url host-supplied), so
lib/commerce stays vendor-agnostic; SumUp/Stripe adapters live in the orders
service and order-settle!(ref, amount) is the resume seam. pending-payments
enumerates suspended orders + envelopes (host poller seam). Gotcha handled: a
Scheme string flow-payload round-trips back wrapped as {:scm-string ...} —
unwrapped via scm->string. Total 209/209 across 13 suites.

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

lib/commerce/scoreboard.md

@@ -12,8 +12,9 @@ _Generated by `lib/commerce/conformance.sh`_
 | stack | 16 | 0 | 16 |
 | quote | 13 | 0 | 13 |
 | ledger | 20 | 0 | 20 |
-| order | 21 | 0 | 21 |
+| order | 22 | 0 | 22 |
 | recon | 20 | 0 | 20 |
 | federation | 12 | 0 | 12 |
 | attribution | 16 | 0 | 16 |
-| **Total** | **201** | **0** | **201** |
+| payment | 7 | 0 | 7 |
+| **Total** | **209** | **0** | **209** |