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

fuzz/fuzz_targets/edit.rs

@@ -9,6 +9,7 @@ use tui_textarea_bench::{dummy_terminal, TerminalExt};
 #[derive(Arbitrary)]
 enum RandomInput {
     Input(Input),
+    InputWithoutShortcuts(Input),
     Cursor(CursorMove),
 }
 
@@ -18,11 +19,30 @@ impl RandomInput {
             Self::Input(input) => {
                 t.input(input);
             }
+            Self::InputWithoutShortcuts(input) => {
+                t.input_without_shortcuts(input);
+            }
             Self::Cursor(m) => t.move_cursor(m),
         }
     }
 }
 
+fn assert_invariants(textarea: &TextArea<'_>) {
+    let lines = textarea.lines();
+    assert!(!lines.is_empty(), "lines must never be empty");
+    let (row, col) = textarea.cursor();
+    assert!(
+        row < lines.len(),
+        "cursor row {row} out of bounds (lines: {})",
+        lines.len()
+    );
+    assert!(
+        col <= lines[row].chars().count(),
+        "cursor col {col} out of bounds (line {row} chars: {})",
+        lines[row].chars().count(),
+    );
+}
+
 fn fuzz(data: &[u8]) -> Result<()> {
     let mut term = dummy_terminal();
     let mut data = Unstructured::new(data);
@@ -32,6 +52,7 @@ fn fuzz(data: &[u8]) -> Result<()> {
         let input = RandomInput::arbitrary(&mut data)?;
         input.apply(&mut textarea);
         term.draw_textarea(&textarea);
+        assert_invariants(&textarea);
     }
     Ok(())
 }