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/wrap_render.rs

@@ -0,0 +1,57 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+use tui_textarea::{TextArea, WrapMode};
+use tui_textarea_bench::{dummy_terminal, TerminalExt, LOREM};
+
+const MODES: &[WrapMode] = &[
+    WrapMode::None,
+    WrapMode::Word,
+    WrapMode::Glyph,
+    WrapMode::WordOrGlyph,
+];
+
+fn make_textarea(repeat: usize, mode: WrapMode) -> TextArea<'static> {
+    let mut lines = Vec::with_capacity(LOREM.len() * repeat);
+    for _ in 0..repeat {
+        lines.extend(LOREM.iter().map(|s| s.to_string()));
+    }
+    let mut ta = TextArea::new(lines);
+    ta.set_wrap_mode(mode);
+    ta
+}
+
+// Short text: 7 lines, each ~63 chars. DummyBackend width=40 → all lines wrap.
+fn short(c: &mut Criterion) {
+    for mode in MODES {
+        let ta = make_textarea(1, *mode);
+        let name = format!("wrap_render::short::{:?}", mode);
+        c.bench_function(&name, |b| {
+            b.iter(|| {
+                let mut term = dummy_terminal();
+                for _ in 0..50 {
+                    term.draw_textarea(&ta);
+                }
+                std::hint::black_box(())
+            })
+        });
+    }
+}
+
+// Long text: 70 lines, same wrapping pressure.
+fn long(c: &mut Criterion) {
+    for mode in MODES {
+        let ta = make_textarea(10, *mode);
+        let name = format!("wrap_render::long::{:?}", mode);
+        c.bench_function(&name, |b| {
+            b.iter(|| {
+                let mut term = dummy_terminal();
+                for _ in 0..50 {
+                    term.draw_textarea(&ta);
+                }
+                std::hint::black_box(())
+            })
+        });
+    }
+}
+
+criterion_group!(wrap_render, short, long);
+criterion_main!(wrap_render);