ui: validity indicator rendering + warning theme colour (ADR-0027 step D)

Adds the `[ERR]` / `[WRN]` validity indicator to the input
row. `App` gains `input_indicator: Option<Severity>` (the
runtime owns its timing — step E) and a pure
`input_validity_verdict()` query that runs `input_verdict`
in simple mode only (advanced mode is raw SQL, ADR-0027 §7).

`render_input_panel` reserves the rightmost six columns of
the input row unconditionally (ADR-0027 §4) — a five-column
label plus a one-column gap — so the typed command never
shifts sideways when the indicator appears or hides. The
label renders only when `input_indicator` is set: `[ERR]` in
`theme.error`, `[WRN]` in the new amber `theme.warning`
(defined for both light and dark themes).

The indicator is not yet wired live — `input_indicator`
stays `None` until the debounce lands (step E). Covered by a
render test and the theme contrast test; the input-panel
snapshot is updated for the six-column reservation.

tests/walking_skeleton.rs

@@ -595,3 +595,23 @@ fn dsl_failure_shows_friendly_error_in_output() {
         "error should include the friendly message:\n{rendered}"
     );
 }
+
+#[test]
+fn validity_indicator_renders_err_and_wrn_labels() {
+    // ADR-0027 §4: the input row shows a `[ERR]` / `[WRN]`
+    // label at its right edge, or nothing when clean.
+    use rdbms_playground::dsl::walker::Severity;
+    let mut app = App::new();
+
+    let clean = rendered_text(&mut app, &Theme::dark(), 80, 24);
+    assert!(!clean.contains("[ERR]"), "clean input shows no label:\n{clean}");
+    assert!(!clean.contains("[WRN]"), "clean input shows no label:\n{clean}");
+
+    app.input_indicator = Some(Severity::Error);
+    let err = rendered_text(&mut app, &Theme::dark(), 80, 24);
+    assert!(err.contains("[ERR]"), "ERROR verdict shows [ERR]:\n{err}");
+
+    app.input_indicator = Some(Severity::Warning);
+    let wrn = rendered_text(&mut app, &Theme::dark(), 80, 24);
+    assert!(wrn.contains("[WRN]"), "WARNING verdict shows [WRN]:\n{wrn}");
+}