feat(hint): H2 Phase C batch 5 — runtime error-class tier-3 hints (ADR-0053)

what/example(fix recipe)/concept for the 9 runtime error classes:
foreign_key parent_side (child_side was the exemplar), unique, not_null,
check, type_mismatch, not_found, already_exists, generic, invalid_value.
Keyed by friendly::error_hint_class; catalogue + keys.rs registered.
+1 spot test; 2496 pass / 1 ignored, clippy clean.

src/app.rs

@@ -5922,6 +5922,17 @@ mod tests {
         assert!(output_contains(&app, "heart of SQL"));
     }
 
+    // ── Phase C batch 5: runtime error-class hints render ───────
+
+    #[test]
+    fn hint_renders_a_runtime_error_block() {
+        let mut app = App::new();
+        app.last_error_hint_key = Some("unique".to_string());
+        type_str(&mut app, "hint");
+        submit(&mut app);
+        assert!(output_contains(&app, "must be unique"));
+    }
+
     #[test]
     fn messages_command_toggles_verbosity_and_reports() {
         let mut app = App::new();