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]).

CLAUDE.md

@@ -89,8 +89,11 @@ Current decisions at a glance (each backed by an ADR):
 - **FK relationships:** declared via `add 1:n relationship [as
   <name>] from <P>.<col> to <C>.<col> [on delete <action>] [on
   update <action>] [--create-fk]`. Implemented through the
-  rebuild-table primitive — the same machinery covers B2's
-  pending column drop/rename/type-change use cases (ADR-0013).
+  rebuild-table primitive — the same machinery backs B2's
+  column drop/rename/type-change operations (ADR-0013), which
+  are implemented in both simple mode (`drop column` /
+  `rename column` / `change column`) and advanced mode
+  (`ALTER TABLE`, ADR-0035 §4e/§4f).
 - **Data operations:** `insert / update / delete / show data`
   with required WHERE plus `--all-rows` opt-in for unfiltered
   ops; auto-show after writes shows just the affected rows;
@@ -192,9 +195,6 @@ not yet implemented:
   1–4 of ADR-0015). Pending pieces: `export` / `import` (Iter
   5), `--resume` + persistent input history hydration +
   migration framework scaffold (Iter 6).
-- **Column drops/renames/type changes** (B2 / C2 partial): the
-  rebuild-table primitive (ADR-0013) is in place; the grammar
-  and dispatch are pending.
 - **Modify relationship** (C3a): drop+add covers the use case
   today.
 - **m:n convenience** (C4): auto-generates a junction table