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/biquad.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Generic second-order IIR biquad, Transposed Direct Form II.
 //!
 //! Coefficient formulas are the RBJ Audio EQ Cookbook