plans: shared-plumbing extraction note — defer to post-merge integration

mod-sx (Prolog) and acl-sx (Datalog) converged on the same module shape but run
on different engines. Only the audit log + fed trust/outbox shapes truly share;
extract at the architecture-merge point refactoring both consumers atomically,
not unilaterally from a loop branch.

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

plans/mod-on-sx.md

@@ -238,6 +238,29 @@ lib/mod/fed.sx
   `mod/dedup-reports` collapses identical reports (reporter|subject|reason key,
   case-insensitive); `mod/distinct-reporters-of` counts unique reporters.
 
+## Shared-plumbing extraction — post-merge integration note
+
+mod-sx (Prolog) and acl-sx (Datalog, `lib/acl/`, 120/120) independently converged
+on the same module shape: `schema / engine / audit / explain / federation / api`.
+That parallel is the signal both plans flagged. **Recommendation: do NOT extract
+from a loop branch — extract at the architecture-merge integration point, after
+both `lib/mod` and `lib/acl` have landed, refactoring both consumers in one change.**
+
+- **Different engines.** acl = Datalog bottom-up (native derivation trees); mod =
+  Prolog backtracking (proof via per-goal `pl-query-all`). The engine and most of
+  `explain` are NOT shared — same intent, different mechanism. Don't try to unify them.
+- **Genuinely convergent shapes (the only real candidates):**
+  - **Append-only audit log** — `{seq, payload, retrieve-by-id}`; both have it (~40
+    lines). Lift to e.g. `lib/guest/audit-log.sx` parameterized by the entry payload.
+  - **Federation trust/outbox** — advisory-unless-`(trust peer :scope)` + a send
+    seam; both have it. Lift the trust registry + outbox; keep `:scope` a parameter
+    (`:mod` vs `:acl`).
+- **Trivia not worth a module:** `join-with`, `any?`, `str-contains?`, `distinct`.
+- **Why not now:** the branches merge independently; lifting from one leaves the
+  other's copy un-refactored → duplication, not sharing. Real extraction must touch
+  both consumers atomically, which only the post-merge integrator can do. Designing
+  the abstraction also needs both payload shapes in view (only mod's is visible here).
+
 ## Progress log
 
 - **Ext 19 — end-to-end triage pipeline, 390/390** (+15). Capstone: one