fix: audit fixes for 4 medium-severity bugs: index_cache tail hash, read_cache doc, mutex poison recovery, Remove event handling

crates/tui/src/app.rs

@@ -848,6 +848,9 @@ impl App {
                     // Don't auto-switch; old content preserved.
                     // User can reload manually if desired.
                 }
+                FileEvent::Removed => {
+                    self.loading_state = AppLoadingState::Error("File has been deleted".into());
+                }
             }
         }
     }
@@ -2651,4 +2654,4 @@ plain text line
         }));
         assert!(result.is_ok());
     }
-}
+}