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/crossover.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! 3-band Linkwitz-Riley (LR4, 24 dB/oct) crossover filterbank.
 //!
 //! Each crossover splits into a low-passed band output and a high-passed remainder that feeds