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/search_backward.rs

@@ -0,0 +1,37 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+use tui_textarea::TextArea;
+use tui_textarea_bench::{dummy_terminal, TerminalExt, LOREM};
+
+#[inline]
+fn run(pat: &str, mut textarea: TextArea<'_>) {
+    let mut term = dummy_terminal();
+    textarea.set_search_pattern(pat).unwrap();
+    term.draw_textarea(&textarea);
+    for _ in 0..100 {
+        textarea.search_back(false);
+        term.draw_textarea(&textarea);
+    }
+    textarea.set_search_pattern(r"").unwrap();
+    term.draw_textarea(&textarea);
+}
+
+fn short(c: &mut Criterion) {
+    let textarea = TextArea::from(LOREM.iter().map(|s| s.to_string()));
+    c.bench_function("search::back_short", |b| {
+        b.iter(|| run(r"\w*i\w*", textarea.clone()))
+    });
+}
+
+fn long(c: &mut Criterion) {
+    let mut lines = vec![];
+    for _ in 0..10 {
+        lines.extend(LOREM.iter().map(|s| s.to_string()));
+    }
+    let textarea = TextArea::new(lines);
+    c.bench_function("search::back_long", |b| {
+        b.iter(|| run(r"[A-Z]\w*", textarea.clone()))
+    });
+}
+
+criterion_group!(search_backward, short, long);
+criterion_main!(search_backward);