commerce: mark roadmap complete + record Phase 5 extension backlog

Base roadmap (Phases 1-4) done at 185/185. Records thesis-aligned extension
candidates (line-level discount attribution, time-windowed promos, discount-aware
tax, refund flow, stock-constrained reservation) for subsequent loop iterations.

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

plans/commerce-on-sx.md

@@ -75,6 +75,21 @@ lib/commerce/api.sx ── (commerce/add) (commerce/total) (commerce/checkout)
 - [x] cross-instance catalog (federated marketplace) — out-of-scope stub
 - [x] tests: webhook replay, partial refund, double-charge guard
 
+## Phase 5 — Extensions (backlog; base roadmap complete)
+Thesis-aligned deepenings of the relational/composition showcase. Pick the one
+that unlocks the most tests per effort each iteration.
+- [ ] line-level discount attribution — "which line item triggered this discount?"
+  as a backward miniKanren query (briefing gotcha; promos currently apply at
+  class level). Refactor promo amounts to also yield per-line attribution.
+- [ ] time-windowed promotions — promos gated by a validity window; quote takes a
+  datetime, determinism preserved. (quote.sx already documents datetime intent.)
+- [ ] discount-aware tax policy — alternative `cart-quote` computing tax on the
+  net (post-discount) base via proportional class allocation; explicit + tested.
+- [ ] refund as a flow — refund lifecycle (request → approve → settle) as a second
+  flow-on-sx flow, recorded in the ledger; idempotent.
+- [ ] stock-constrained reservation — order-begin! fails (railway `fail`) when
+  requested qty exceeds stocko availability; reservation decrements a stock view.
+
 ## Progress log
 - 2026-06-07 — `recon.sx` + `federation.sx` (**Phase 4 complete — roadmap done**).
   `recon.sx`: reconciliation as relational queries over the ledger. Per-order