feat: DSL→SQL teaching echo — Phase 3 cat-3 caveat (ADR-0038)

Lands the only piece of category-3 prose not already covered by the
existing `client_side.*` notes infrastructure: the `change column …
--dont-convert` *caveat* (ADR-0038 §6, the only Bucket A caveat —
every other category-3 line is illuminating).

`--dont-convert` skips the client-side layer entirely, so the headline
SQL echo (`ALTER TABLE … SET DATA TYPE …`) is the nearest SQL but
*not* equivalent: running the line in advanced mode would convert the
stored values, but the playground left them as-is. The new caveat
states that divergence explicitly.

* New i18n key `client_side.dont_convert_caveat` (no placeholders) —
  registered in keys::KEYS_AND_PLACEHOLDERS.
* New `dont_convert_caveat: bool` field on DslChangeColumnSucceeded,
  set in the runtime when submission_mode is advanced *and* the
  command is ChangeColumnType { mode: DontConvert, .. }. Gated on
  advanced mode because the caveat references "the line above" — the
  echo, which only fires in advanced mode.
* App's handle_dsl_change_column_success emits the caveat line
  between the existing client-side notes and the structure render,
  so it reads alongside the echo, not after the table view.

The other two category-3 lines from §6 (shortid generation,
type-conversion transforms) were already in place via
`client_side.auto_fill_*` / `client_side.transformed*` — those notes
already render after the echo via handle_dsl_add_column_success /
handle_dsl_change_column_success, in the right position per the ADR.
This commit just adds the missing caveat.

Tests: 2014 passed / 0 failed / 1 ignored (pre-existing); clippy
clean. An App-level test pins the rendering order (caveat sits
after the echo, before the structure) and the simple-mode gate
(no caveat without an echo to refer to).

The §4 de-emphasised styled-runs rendering polish remains —
the echo + caveat lines are still plain `[system]` lines.

src/runtime.rs

@@ -1344,11 +1344,28 @@ fn spawn_dsl_dispatch(
                 result,
                 echo,
             },
-            Ok(CommandOutcome::ChangeColumn(result)) => AppEvent::DslChangeColumnSucceeded {
-                command: command.clone(),
-                result,
-                echo,
-            },
+            Ok(CommandOutcome::ChangeColumn(result)) => {
+                // ADR-0038 §6 category 3 caveat: `--dont-convert` skips
+                // the client-side layer entirely, so the headline echo
+                // (`ALTER TABLE … SET DATA TYPE …`) is the *nearest*
+                // SQL but not equivalent. The caveat only makes sense
+                // next to the echo, so gate on advanced mode (the line
+                // references "the line above").
+                let dont_convert_caveat = submission_mode.is_advanced()
+                    && matches!(
+                        &command,
+                        Command::ChangeColumnType {
+                            mode: ChangeColumnMode::DontConvert,
+                            ..
+                        }
+                    );
+                AppEvent::DslChangeColumnSucceeded {
+                    command: command.clone(),
+                    result,
+                    echo,
+                    dont_convert_caveat,
+                }
+            }
             Ok(CommandOutcome::AddColumn(result)) => AppEvent::DslAddColumnSucceeded {
                 command: command.clone(),
                 result,