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- Delete legacy magic.rs (unused, no feature gating, superseded by magic_file.rs)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/tests/compression/lz4_tests.rs

@@ -1,4 +1,5 @@
 #[cfg(test)]
+#[cfg(feature = "lz4")]
 mod tests {
     use crate::compression_engine::lz4::CompressionEngineLZ4;
     use crate::tests::common::test_helpers::test_compression_engine;