fix(bench): wrap file writes with BufWriter to reduce syscall overhead

Add BufWriter::with_capacity(64KB) to generate_test_file,
generate_growable_file, and append_lines in data_gen.rs.

Previously each writeln! triggered an individual write syscall,
making 5GB/74M-line benchmark data generation extremely slow.
BufWriter batches writes into 64KB chunks, reducing syscalls
by ~1000x.

Explicit flush()? + drop before subsequent reads ensures data
visibility and propagates flush errors (BufWriter::drop swallows
them).

Closes #35

crates/bench/src/data_gen.rs

@@ -1,5 +1,5 @@
 use std::fs;
-use std::io::{BufRead, BufReader, Write};
+use std::io::{BufRead, BufReader, BufWriter, Write};
 use std::path::{Path, PathBuf};
 
 pub struct TestFileInfo {
@@ -51,7 +51,7 @@ fn generate_test_file(path: &Path) -> std::io::Result<TestFileInfo> {
         fs::create_dir_all(parent)?;
     }
 
-    let mut file = fs::File::create(path)?;
+    let mut file = BufWriter::with_capacity(64 * 1024, fs::File::create(path)?);
     let target_lines: u64 = 74_000_000;
     for i in 0..target_lines {
         writeln!(
@@ -63,6 +63,8 @@ fn generate_test_file(path: &Path) -> std::io::Result<TestFileInfo> {
             i * 7
         )?;
     }
+    file.flush()?;
+    drop(file);
 
     get_file_info(path)
 }
@@ -72,7 +74,7 @@ pub fn generate_growable_file(dir: &Path) -> std::io::Result<PathBuf> {
     fs::create_dir_all(dir)?;
     let path = dir.join("growable.log");
 
-    let mut file = fs::File::create(&path)?;
+    let mut file = BufWriter::with_capacity(64 * 1024, fs::File::create(&path)?);
     for i in 0..150_000u64 {
         writeln!(
             file,
@@ -82,13 +84,17 @@ pub fn generate_growable_file(dir: &Path) -> std::io::Result<PathBuf> {
             i
         )?;
     }
+    file.flush()?;
 
     Ok(path)
 }
 
 /// Append `count` lines to the file
 pub fn append_lines(path: &Path, count: usize) -> std::io::Result<()> {
-    let mut file = fs::OpenOptions::new().append(true).open(path)?;
+    let mut file = BufWriter::with_capacity(
+        64 * 1024,
+        fs::OpenOptions::new().append(true).open(path)?,
+    );
     let existing_lines = count_existing_lines(path).unwrap_or(0);
     for i in 0..count {
         writeln!(
@@ -97,6 +103,7 @@ pub fn append_lines(path: &Path, count: usize) -> std::io::Result<()> {
             existing_lines + i as u64
         )?;
     }
+    file.flush()?;
     Ok(())
 }