feat: optimization refactor, test suite, and dependency cleanup

Refactored the application to improve performance, maintainability, and binary size.

Key Changes:
- **Core Algorithm**: Switched to a parallel, single-pass tree construction algorithm for O(N) traversal.
- **Architecture**: Extract core logic from `main.rs` to `lib.rs` to enable unit testing.
- **Testing**: Added comprehensive unit tests and integration tests (`tests/cli.rs`) covering CLI arguments and output formats.
- **Benchmarking**: Added `criterion` benchmarks for performance tracking.
- **Dependencies**:
  - Removed `walkdir` (logic replaced by `ignore` + custom tree).
  - Removed `humansize` (consolidated to `byte-unit`).
  - Replaced `chrono` with lightweight `time` crate.
  - Removed unused `atty`.
- **Reporting**: Optimized CSV and JSON output for correctness and zero-copy performance.
- **Documentation**: Updated CHANGELOG.md and bumped version to 0.2.0.

benches/benchmark.rs

@@ -0,0 +1,34 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use sized::build_tree;
+use std::fs::{self, File};
+use std::io::Write;
+use tempfile::TempDir; // Ensure this is pub in lib.rs
+
+fn setup_test_dir() -> TempDir {
+    let temp_dir = TempDir::new().unwrap();
+    let base_path = temp_dir.path();
+
+    // Create a moderately deep structure
+    for i in 0..10 {
+        let dir_path = base_path.join(format!("dir_{}", i));
+        fs::create_dir(&dir_path).unwrap();
+        for j in 0..10 {
+            let file_path = dir_path.join(format!("file_{}.txt", j));
+            let mut file = File::create(file_path).unwrap();
+            writeln!(file, "Some content for file {}-{}", i, j).unwrap();
+        }
+    }
+    temp_dir
+}
+
+fn criterion_benchmark(c: &mut Criterion) {
+    let temp_dir = setup_test_dir();
+    let path = temp_dir.path();
+
+    c.bench_function("build_tree small", |b| {
+        b.iter(|| build_tree(black_box(path), black_box(false)))
+    });
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);