docs(adr): design the DSL→SQL teaching echo (ADR-0038) + dependencies

Realises ADR-0030 §10 (the DSL→SQL teaching bridge) as a /runda'd design
set, before implementation:

- ADR-0037 (new): execution-time mode side-channel — SubmissionMode
  {Simple, Advanced, AdvancedOneShot} threaded Action→worker, output-only;
  redeems ADR-0033 Amendment 3's deferred follow-up. Replay stays silent.
- ADR-0038 (new): the teaching echo + full catalogue (Buckets A/B/C),
  the copy-paste round-trip contract, the three-category framework, and
  the Value→SQL-literal renderer. DDL + show-data centric (overlapping
  DML is SQL-first, so already SQL). Build-order deps recorded.
- ADR-0035 Amendment 2: standard-first dialect stance + ALTER COLUMN
  SET/DROP NOT NULL, SET/DROP DEFAULT, ISO SET DATA TYPE gap-fill.
- ADR-0033 Amendment 4: reclassifies the `update … --all-rows`
  non-fall-back as a bug; it now falls back to the DSL Update and echoes
  (keyed on adjacent `--`; spaced arithmetic preserved).
- ADR-0039 (new): EXPLAIN over advanced SQL — decision recorded, build
  deferred; supersedes ADR-0030 §13 OOS-2.
- ADR-0000: out-of-scope discipline (deferred vs rejected). README index
  updated for all of the above.

Reconcile CLAUDE.md: simple-mode column ops are implemented, not pending
(requirements.md C2/B2 already [x]).

docs/adr/0030-advanced-mode-sql-surface.md

@@ -301,6 +301,9 @@ until then.
   by the items panel's design (`S2`) but need their own model.
 - **OOS-2.** `EXPLAIN` of advanced-mode SQL queries. The DSL
   `explain` (ADR-0028) still works for what it already wraps.
+  **(Superseded by ADR-0039, 2026-05-27 — this was a *deferred*
+  scope exclusion, not a principled rejection; EXPLAIN over advanced
+  SQL is now in scope, as a deferred follow-up.)**
 - **OOS-3.** A function/expression allowlist for full
   expression-level engine neutrality (§7) — best-effort now.
 - **OOS-4.** Multi-statement batches and transaction control.