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/event.rs

@@ -27,6 +27,12 @@ pub enum AppEvent {
     DslSucceeded {
         command: Command,
         description: Option<TableDescription>,
+        /// The DSL → SQL teaching echo (ADR-0038): equivalent advanced-mode
+        /// SQL, built by the runtime when a DSL-form command ran in an
+        /// advanced effective mode (ADR-0037). `None` when no echo applies
+        /// (simple mode, SQL-entered, or a form with no echo). The App
+        /// renders it beneath `[ok]`.
+        echo: Option<String>,
     },
     /// A SQL `CREATE TABLE IF NOT EXISTS` matched an existing table —
     /// a no-op (ADR-0035 §4). Renders the existing structure plus an