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

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

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

src/dsp/limiter.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Look-ahead brickwall peak limiter (base-rate).
 //!
 //! Guarantees the output never exceeds the ceiling. A short fixed look-ahead lets the gain ramp

src/dsp/mod.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! DSP building blocks for Codename 206.
 //!
 //! The signal chain: a `crossover` filterbank splits into bands, each band (plus the summed

src/dsp/oversampler.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! 4× polyphase interpolation for **true-peak (inter-sample) detection only**.
 //!
 //! A band-limited signal can overshoot its sample values between samples, so the digital sample