feat: DSL→SQL teaching echo — §4 styled-runs polish (ADR-0038)

Lands the last open item on ADR-0038: the de-emphasised styled-runs
rendering treatment for the echo + every category-3 prose line. The
echoed SQL now reads as code — the dimmed `Executing SQL:` label
plus the SQL portion lexed and coloured the same way the input echo
treats user-typed input (ADR-0028 §5 styled-runs over
input_render::lex_to_runs in advanced mode). Category-3 prose lines
(the DontConvert caveat and the existing illuminating
`client_side.*` notes — shortid auto-fill, type-conversion
transforms) all render dimmed too, per §6's "de-emphasised prose
line" wording, so every cat-3 line is visually consistent.

* New `OutputKind::TeachingEcho` variant + a custom branch in
  `ui::render_output_line` mirroring the OutputKind::Echo input-echo
  path: strip the canonical `Executing SQL:` prefix, render it with
  `theme.muted`, then lex the rest in `Mode::Advanced` and emit one
  span per token. Tag stays `[system]` for visual consistency with
  other system output.
* New `OutputStyleClass::Hint` styled-runs class, resolved to
  `theme.muted` in `output_span_style`. Used for the cat-3 prose
  lines (dont_convert caveat + the existing client_side notes).
* New const `crate::echo::TEACHING_ECHO_LABEL = "Executing SQL: "` —
  the byte boundary the ui.rs branch needs is fixed (an i18n template
  can't provide that), so the label moves from i18n to a constant.
  The `echo.executing_sql` i18n key is retired (en-US.yaml + keys.rs);
  a comment in en-US.yaml points future locales at re-introducing it
  if needed.
* App-side helpers: `push_teaching_echo(sql)` builds the
  TeachingEcho line; `push_category_three_prose(text)` builds a
  System line with a whole-text Hint span. `note_ok_summary` and
  `handle_dsl_change_column_success` / `handle_dsl_add_column_success`
  use these instead of plain `note_system` for the echo, the caveat,
  and the illuminating notes.

Existing tests pass unchanged — text content is the same; only
styling changes. New tests pin the polish:

* `ui::tests::teaching_echo_line_renders_dim_prefix_and_lexed_sql`
  asserts the TeachingEcho rendering produces a dim prefix span +
  keyword-coloured SQL spans (confirming the lexer ran in advanced
  mode).
* `ui::tests::category_three_prose_line_renders_all_dim` pins the
  whole-text Hint coverage.
* `ui::tests::hint_class_resolves_to_muted_foreground` pins the
  theme resolution across both light and dark.
* `app::tests::polished_echo_carries_teaching_echo_kind_and_caveat_a_hint_span`
  pins the App-side wiring (kind + styled_runs shape).

Tests: 2019 passed / 0 failed / 1 ignored (pre-existing); clippy
clean (`--all-targets -D warnings`, nursery).

ADR-0038 is now feature-complete — every catalogue row implemented,
round-tripped, AND polished per §4.

src/echo.rs

@@ -21,6 +21,17 @@ use crate::dsl::command::{
 };
 use crate::dsl::value::Value;
 
+/// The dimmed `Executing SQL:` prefix on a teaching-echo line
+/// (ADR-0038 §4 styled-runs polish).
+///
+/// The App appends the SQL to this prefix when building a
+/// `OutputKind::TeachingEcho` line; `ui::render_output_line` strips
+/// it back off, renders the prefix dimmed (`theme.muted`), and lexes
+/// the rest in advanced mode for syntax highlighting — same treatment
+/// the input echo gets. The trailing space is part of the constant so
+/// the SQL position is predictable for the lexer's byte ranges.
+pub const TEACHING_ECHO_LABEL: &str = "Executing SQL: ";
+
 /// The teaching echo for a command submitted under `mode`, or `None`.
 ///
 /// Fires only in an advanced effective mode (`AdvancedPersistent` /