chore(fuzz): upgrade targets, fix false positives, and add invariant assertions (#11)

- Fix insert_delete target using .unwrap() causing false-positive crashes
  on arbitrary::Error (input exhaustion)
- Fix fuzz Cargo.toml referencing wrong package name (tui-textarea vs
  tui-textarea-2)
- Add invariant assertions (cursor bounds, lines non-empty, selection
  validity) to all targets
- Add 4 new fuzz targets: undo_redo, selection, wrap_render, search
- Expand existing targets with full delete/insert variant coverage,
  input_without_shortcuts, set_yank_text, set_max_histories, and clear
- Add DummyBackend::resize() for variable-width wrap rendering tests

bench/src/lib.rs

@@ -123,6 +123,14 @@ impl Backend for DummyBackend {
     }
 }
 
+impl DummyBackend {
+    #[inline]
+    pub fn resize(&mut self, width: u16, height: u16) {
+        self.width = width;
+        self.height = height;
+    }
+}
+
 #[inline]
 pub fn dummy_terminal() -> Terminal<DummyBackend> {
     Terminal::new(DummyBackend::default()).unwrap()