feat: DSL→SQL teaching echo — channel + create-table slice (ADR-0037 + ADR-0038)

Walking skeleton validating the whole echo architecture end to end; the
Command→SQL renderer currently covers `create table`, with the rest of
Bucket A / B / category-3 to follow (ADR-0038 §8).

- Channel (ADR-0037): the three-way EffectiveMode (reusing the existing
  enum, not a new SubmissionMode — recorded in the ADR) rides on
  Action::ExecuteDsl to the runtime. `replay` bypasses the interactive
  spawn, so it never echoes (silent, for free).
- Echo (ADR-0038): built at the runtime's ExecuteDsl dispatch — the worker
  gets decomposed calls, not the Command, so ADR §4's "worker builds it"
  was corrected to the dispatch layer. Gated by echo_for (advanced
  effective mode + DSL-form). Carried on DslSucceeded; rendered by
  note_ok_summary as `Executing SQL: …` immediately beneath `[ok]`. New
  src/echo.rs renderer; echo.executing_sql i18n key.
- command_to_sql: `create table` → `CREATE TABLE T (id serial PRIMARY KEY)`
  (single inline / compound table-level PK), playground type vocabulary,
  round-trip-verified against the advanced walker (the §1 contract).

Tests: echo.rs (render, round-trip contract, mode gate, Sql*-not-echoed);
app.rs (submit carries the 3-way mode; echo renders beneath [ok]).
Suite 1970/0/1; clippy clean.

src/friendly/strings/en-US.yaml

@@ -874,6 +874,12 @@ db:
       Cannot add this CHECK to `{table}.{column}`: {total} row(s) do not satisfy `{rule}`.
 
 # ---- DSL command success summaries (ADR-0019 §9 sweep) --------------
+# DSL → SQL teaching echo (ADR-0038): the equivalent advanced-mode SQL,
+# rendered beneath `[ok]` for a DSL-form command run in an advanced
+# effective mode (ADR-0037).
+echo:
+  executing_sql: "Executing SQL: {sql}"
+
 ok:
   # Generic `[ok] <verb> <subject>` header used for every
   # successful DSL command. Verbs come from `Command::verb()`