refactor(bench): unify line counting in get_file_info to use count_existing_lines

Reuse the existing count_existing_lines() (reader.lines().count())
instead of a manual read_until loop, eliminating duplicate line-counting
logic in data_gen.rs.

Closes #40

crates/bench/src/data_gen.rs

@@ -21,15 +21,7 @@ pub fn ensure_test_file(path: &Path) -> std::io::Result<TestFileInfo> {
 fn get_file_info(path: &Path) -> std::io::Result<TestFileInfo> {
     let metadata = fs::metadata(path)?;
     let size_bytes = metadata.len();
-
-    let file = fs::File::open(path)?;
-    let mut reader = BufReader::new(file);
-    let mut line_count: u64 = 0;
-    let mut buf = Vec::new();
-    while reader.read_until(b'\n', &mut buf)? > 0 {
-        line_count += 1;
-        buf.clear();
-    }
+    let line_count = count_existing_lines(path)?;
 
     let avg_line_length = if line_count > 0 {
         size_bytes as f64 / line_count as f64