commerce: order lifecycle as a durable flow-on-sx flow (21 tests) — Phase 3 done

order.sx — reserve -> await-payment -> fulfil as a flow-on-sx flow carrying
only the order-id; the SX driver services each request by appending to the
persist ledger. order-begin! creates+reserves and suspends at payment;
order-settle! (webhook) resumes -> fulfils, idempotent on replay
(:already-settled). order-flow-restart! simulates a process restart Scheme-side
and the suspended order resumes with the ledger intact. Composes all three
substrates: minikanren pricing -> flow lifecycle -> persist ledger.
Total 153/153 across 9 suites.

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

lib/commerce/scoreboard.md

@@ -12,4 +12,5 @@ _Generated by `lib/commerce/conformance.sh`_
 | stack | 16 | 0 | 16 |
 | quote | 13 | 0 | 13 |
 | ledger | 20 | 0 | 20 |
-| **Total** | **132** | **0** | **132** |
+| order | 21 | 0 | 21 |
+| **Total** | **153** | **0** | **153** |