runtime: debounce the validity indicator (ADR-0027 step E)

The event loop now time-boxes `recv` while an indicator
recompute is owed: every keystroke hides the indicator and
arms an `INDICATOR_DEBOUNCE` (1s) window; once typing pauses
that long the runtime computes `App::input_validity_verdict`
and shows `[ERR]` / `[WRN]`. An idle session (nothing owed)
still blocks plainly on `recv` — no wake-ups.

`update()` stays pure — the debounce timer lives in the
runtime; `App` only holds the resulting `input_indicator`
state, which the runtime clears on a keystroke and sets when
the quiet interval elapses.

`App::input_validity_verdict` is tested directly (a
simple-mode verdict, and silence in advanced mode / the `:`
one-shot); the debounce timing itself is runtime-loop glue,
covered at the integration level.

src/app.rs

@@ -3042,4 +3042,41 @@ mod tests {
             },
         );
     }
+
+    // ---- Validity-indicator verdict (ADR-0027) ----------------
+
+    #[test]
+    fn input_validity_verdict_flags_a_broken_simple_command() {
+        let mut app = App::new();
+        app.input = "create table".to_string();
+        assert_eq!(
+            app.input_validity_verdict(),
+            Some(crate::dsl::walker::Severity::Error),
+        );
+    }
+
+    #[test]
+    fn input_validity_verdict_is_none_for_clean_input() {
+        let mut app = App::new();
+        app.input = "quit".to_string();
+        assert_eq!(app.input_validity_verdict(), None);
+    }
+
+    #[test]
+    fn input_validity_verdict_silent_in_advanced_mode() {
+        // Advanced mode is raw SQL — the DSL walker must not
+        // pass a verdict on it (ADR-0027 §7).
+        let mut app = App::new();
+        app.mode = Mode::Advanced;
+        app.input = "create table".to_string();
+        assert_eq!(app.input_validity_verdict(), None);
+    }
+
+    #[test]
+    fn input_validity_verdict_silent_for_colon_one_shot() {
+        // A `:`-prefixed line is a one-shot advanced escape.
+        let mut app = App::new();
+        app.input = ":create table".to_string();
+        assert_eq!(app.input_validity_verdict(), None);
+    }
 }