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/0000-record-architecture-decisions.md

@@ -38,6 +38,25 @@ The index lists ADRs in numerical order. Each entry shows the
 number, title, and — where relevant — status annotations such as
 "Superseded by ADR-NNNN" or "Deprecated".
 
+## Out-of-scope discipline
+
+ADRs (and the plans they spawn) lean heavily on "out of scope" language.
+The phrase carries two very different meanings, and conflating them
+misleads a later reader:
+
+- **Deferred** — out of scope *for this plan / phase / step*, but a
+  reasonable thing to do later. A sequencing decision, effectively a
+  tracked TODO. Where possible, point at where it will be picked up.
+- **Rejected** — considered and deliberately *not* done, on principle.
+  Durable. State the reason.
+
+When writing an out-of-scope item, **say which kind it is** — e.g.
+`OOS (deferred)` / `OOS (rejected: <reason>)`, or the equivalent in prose
+— so a future reader can tell a standing decision from a not-yet. A bare
+"out of scope" is ambiguous and tends to read, wrongly, as permanent.
+(Motivating example: ADR-0030 §13 OOS-2 was a deferred scope exclusion
+that read as a permanent rejection until ADR-0039 lifted it.)
+
 ## Consequences
 
 - New significant decisions require an ADR before or alongside the