feat(hint): H2 Phase C batch 1 — app-command tier-3 hints (ADR-0053)

Per-form hints for the 14 app-lifecycle commands (quit/help/hint/
rebuild/save/new/load/export/import/mode/messages/undo/redo/copy),
reference-leaning what/example with concept where it teaches (rebuild,
mode, messages, undo, export, help). hint_ids wired, catalogue + keys.rs
registered. +1 spot test; 2489 pass / 1 ignored, clippy clean.

src/app.rs

@@ -5849,6 +5849,19 @@ mod tests {
         );
     }
 
+    // ── Phase C batch 1: app-command hints render ───────────────
+
+    #[test]
+    fn f1_on_an_app_command_renders_its_hint_block() {
+        let mut app = App::new();
+        type_str(&mut app, "mode advanced");
+        f1(&mut app);
+        assert!(
+            output_contains(&app, "Switch between simple mode"),
+            "expected the `mode` tier-3 block"
+        );
+    }
+
     #[test]
     fn messages_command_toggles_verbosity_and_reports() {
         let mut app = App::new();