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/insert_append.rs

@@ -0,0 +1,57 @@
+use criterion::{criterion_group, criterion_main, Criterion, SamplingMode};
+use tui_textarea::{Input, Key, TextArea};
+use tui_textarea_bench::{dummy_terminal, TerminalExt, LOREM};
+
+#[inline]
+fn append_lorem(repeat: usize) -> usize {
+    let mut textarea = TextArea::default();
+    let mut term = dummy_terminal();
+    for _ in 0..repeat {
+        for line in LOREM {
+            for c in line.chars() {
+                textarea.input(Input {
+                    key: Key::Char(c),
+                    ctrl: false,
+                    alt: false,
+                    shift: false,
+                });
+                term.draw_textarea(&textarea);
+            }
+        }
+        textarea.input(Input {
+            key: Key::Enter,
+            ctrl: false,
+            alt: false,
+            shift: false,
+        });
+        term.draw_textarea(&textarea);
+    }
+    textarea.lines().len()
+}
+
+fn bench(c: &mut Criterion) {
+    let mut group = c.benchmark_group("insert::append");
+
+    // ~16ms/iter — fits 100 samples in ~6s, keep defaults
+    group.bench_function("1_lorem", |b| {
+        b.iter(|| std::hint::black_box(append_lorem(1)))
+    });
+
+    // ~490ms/iter — 20 samples × 490ms ≈ 10s
+    group.sampling_mode(SamplingMode::Flat);
+    group.sample_size(20);
+    group.bench_function("10_lorem", |b| {
+        b.iter(|| std::hint::black_box(append_lorem(10)))
+    });
+
+    // ~5.9s/iter — 10 samples × 5.9s ≈ 59s (minimum viable)
+    group.sample_size(10);
+    group.bench_function("50_lorem", |b| {
+        b.iter(|| std::hint::black_box(append_lorem(50)))
+    });
+
+    group.finish();
+}
+
+criterion_group!(insert_append, bench);
+criterion_main!(insert_append);