chore(bench): restructure into focused targets, add runner, and tune slow cases (#10)

- Upgrade Criterion 0.5 → 0.8.2
- Split 4 broad bench files into 14 focused targets
- Add Rust runner binary with per-target start/done logging
- Add wrap_render benchmarks (all 4 WrapMode variants × 2 sizes)
- Add undo_redo benchmarks (undo/redo × 3 history depths)
- Apply benchmark_group + SamplingMode::Flat + reduced sample_size on slow insert cases to cut total run time from ~28 min to ~12 min
- Rewrite bench/README.md to document runner, targets, and workflow

bench/benches/undo_redo.rs

@@ -0,0 +1,66 @@
+use criterion::{criterion_group, criterion_main, Criterion, SamplingMode};
+use tui_textarea::TextArea;
+use tui_textarea_bench::{dummy_terminal, TerminalExt};
+
+/// Build a textarea with `depth` undo-able edits in history using insert_str,
+/// interspersed with delete_str to cover the cursor-clamping code path.
+fn make_history(depth: usize) -> TextArea<'static> {
+    let mut ta = TextArea::default();
+    for i in 0..depth {
+        if i % 4 == 3 {
+            // Every 4th edit is a delete_str — the operation fixed in e6cc8ab.
+            ta.delete_str(3);
+        } else {
+            ta.insert_str("hello ");
+        }
+    }
+    ta
+}
+
+#[inline]
+fn undo_all(textarea: &TextArea<'_>) -> usize {
+    let mut t = textarea.clone();
+    let mut term = dummy_terminal();
+    let mut count = 0;
+    while t.undo() {
+        term.draw_textarea(&t);
+        count += 1;
+    }
+    count
+}
+
+#[inline]
+fn redo_all(textarea: &TextArea<'_>) -> usize {
+    // Start from fully-undone state so redo has work to do.
+    let mut t = textarea.clone();
+    while t.undo() {}
+    let mut term = dummy_terminal();
+    let mut count = 0;
+    while t.redo() {
+        term.draw_textarea(&t);
+        count += 1;
+    }
+    count
+}
+
+fn bench(c: &mut Criterion) {
+    let mut group = c.benchmark_group("undo_redo");
+    // undo::50 ran Linear with R²=0.19 (Criterion warned "enable flat sampling").
+    // All depths benefit from Flat since history-clone cost varies with depth.
+    group.sampling_mode(SamplingMode::Flat);
+
+    for depth in [50usize, 200, 500] {
+        let ta = make_history(depth);
+        group.bench_function(format!("undo::{}", depth), |b| {
+            b.iter(|| std::hint::black_box(undo_all(&ta)))
+        });
+        group.bench_function(format!("redo::{}", depth), |b| {
+            b.iter(|| std::hint::black_box(redo_all(&ta)))
+        });
+    }
+
+    group.finish();
+}
+
+criterion_group!(undo_redo, bench);
+criterion_main!(undo_redo);