feat: add lz4 command fallback, remove unused magic.rs

- Add program-based lz4 command fallback when lz4 feature is disabled
- Feature-gate lz4.rs and lz4 tests to compile without lz4_flex
- Delete legacy magic.rs (unused, no feature gating, superseded by magic_file.rs)

src/compression_engine/program.rs

@@ -1,4 +1,4 @@
-use anyhow::{Context, Result, anyhow};
+use anyhow::{anyhow, Context, Result};
 use log::*;
 use std::fs::File;
 use std::io::{Read, Write};