feat(hint): H2 Phase D — coverage gate, F1 strip, status flips (ADR-0053)

Completes H2:
- comprehensiveness coverage tests: every REGISTRY command form has a
  hint_id resolving to a hint.cmd.* block, and every runtime error class
  resolves to a hint.err.* block (enforces ADR-0053 D6)
- ADR-0051 keybinding strip advertises F1 in the editing (leads) and
  default states; +shortcut.hint label; 12 full-panel snapshots
  re-accepted (status-bar line only)
- flip ADR-0053 -> implemented, requirements H2 + A1 -> [x], README

2498 pass / 1 ignored, clippy clean.

src/ui.rs

@@ -1894,22 +1894,25 @@ fn status_bar_bindings(app: &App) -> Vec<(&'static str, String)> {
             ("Enter", crate::t!("shortcut.run")),
         ];
     }
-    // 4. Editing — the input has text: surface the readline edit keys
-    //    (ADR-0049). The highest-value subset stays within the width
-    //    budget; Ctrl-K/U remain unadvertised muscle memory.
+    // 4. Editing — the input has text: F1 (the contextual hint for what
+    //    you're typing, ADR-0053) leads, then the readline edit keys
+    //    (ADR-0049). Ctrl-K/U remain unadvertised muscle memory.
     if !app.input.is_empty() {
         return vec![
+            ("F1", crate::t!("shortcut.hint")),
             ("Esc", crate::t!("shortcut.clear")),
             ("Ctrl-A/E", crate::t!("shortcut.home_end")),
             ("Ctrl-W", crate::t!("shortcut.del_word")),
             ("Enter", crate::t!("shortcut.run")),
         ];
     }
-    // 5. Default — empty input, Input focus.
+    // 5. Default — empty input, Input focus. F1 here expands on the most
+    //    recent error, or points the user at getting started (ADR-0053).
     vec![
         ("Ctrl-O", crate::t!("shortcut.nav")),
         ("Tab", crate::t!("shortcut.complete")),
         ("↑", crate::t!("shortcut.history")),
+        ("F1", crate::t!("shortcut.hint")),
         ("Enter", crate::t!("shortcut.run")),
     ]
 }
@@ -2664,7 +2667,7 @@ mod tests {
     #[test]
     fn strip_default_state_is_nav_complete_history_run() {
         let app = App::new();
-        assert_eq!(strip_keys(&app), vec!["Ctrl-O", "Tab", "↑", "Enter"]);
+        assert_eq!(strip_keys(&app), vec!["Ctrl-O", "Tab", "↑", "F1", "Enter"]);
     }
 
     #[test]
@@ -2675,7 +2678,7 @@ mod tests {
         app.input.push_str("create ta");
         assert_eq!(
             strip_keys(&app),
-            vec!["Esc", "Ctrl-A/E", "Ctrl-W", "Enter"],
+            vec!["F1", "Esc", "Ctrl-A/E", "Ctrl-W", "Enter"],
         );
     }