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/cursor_char.rs

@@ -0,0 +1,50 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+use tui_textarea::CursorMove;
+use tui_textarea_bench::{prepare_cursor_textarea, run_cursor, Restore};
+
+fn bench(c: &mut Criterion) {
+    let textarea = prepare_cursor_textarea();
+    c.bench_function("cursor::char::forward", |b| {
+        b.iter(|| {
+            std::hint::black_box(run_cursor(
+                textarea.clone(),
+                &[CursorMove::Forward],
+                Restore::TopLeft,
+                1000,
+            ))
+        })
+    });
+    c.bench_function("cursor::char::back", |b| {
+        b.iter(|| {
+            std::hint::black_box(run_cursor(
+                textarea.clone(),
+                &[CursorMove::Back],
+                Restore::BottomRight,
+                1000,
+            ))
+        })
+    });
+    c.bench_function("cursor::char::down", |b| {
+        b.iter(|| {
+            std::hint::black_box(run_cursor(
+                textarea.clone(),
+                &[CursorMove::Down],
+                Restore::TopLeft,
+                1000,
+            ))
+        })
+    });
+    c.bench_function("cursor::char::up", |b| {
+        b.iter(|| {
+            std::hint::black_box(run_cursor(
+                textarea.clone(),
+                &[CursorMove::Up],
+                Restore::BottomLeft,
+                1000,
+            ))
+        })
+    });
+}
+
+criterion_group!(cursor_char, bench);
+criterion_main!(cursor_char);