Switch license from GPL-3.0 to Apache-2.0

- Replace LICENSE with properly filled Apache-2.0 (Mikkeli Matlock 2026)
- Update Cargo.toml license field
- Add SPDX-License-Identifier headers to all .rs source files

src/dsp/compressor.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Single full-band feed-forward compressor with look-ahead.
 //!
 //! Design follows Giannoulis, Massberg & Reiss, "Digital Dynamic Range Compressor