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.

src/ui.rs

@@ -643,8 +643,37 @@ fn render_input_panel(app: &App, theme: &Theme, frame: &mut Frame<'_>, area: Rec
             plain_input_spans(&app.input, cursor, theme)
         }
     };
-    let paragraph = Paragraph::new(Line::from(spans)).block(block);
-    frame.render_widget(paragraph, area);
+    // ADR-0027 §4: the rightmost six columns of the input row
+    // (a five-column label plus a one-column gap) are reserved
+    // unconditionally, so the text area is always
+    // `inner.width - 6` and the typed command never shifts
+    // sideways when the validity indicator appears or hides.
+    let inner = block.inner(area);
+    frame.render_widget(block, area);
+    let text_area = Rect {
+        width: inner.width.saturating_sub(6),
+        ..inner
+    };
+    frame.render_widget(Paragraph::new(Line::from(spans)), text_area);
+
+    if let Some(severity) = app.input_indicator {
+        let (indicator_label, color) = match severity {
+            crate::dsl::walker::Severity::Error => ("[ERR]", theme.error),
+            crate::dsl::walker::Severity::Warning => ("[WRN]", theme.warning),
+        };
+        let label_area = Rect {
+            x: inner.x + inner.width.saturating_sub(5),
+            width: 5.min(inner.width),
+            ..inner
+        };
+        frame.render_widget(
+            Paragraph::new(Line::from(Span::styled(
+                indicator_label,
+                Style::default().fg(color).add_modifier(Modifier::BOLD),
+            ))),
+            label_area,
+        );
+    }
 }
 
 /// Convert `StyledRun`s into ratatui `Span`s borrowed from