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

src/editor/crossover.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Draggable crossover handles — replaces the two plain crossover sliders.
 //!
 //! A horizontal log-frequency strip split into LOW / MID / HIGH by two draggable handles, plus a

src/editor/gain_curve.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Static gain-curve display: output level vs input level for the channel currently selected in
 //! the plot. Plots the wet transfer `out = (in + pre_gain) + gain_reduction(...) + makeup` (mix not
 //! folded in; output clamped at 0 dBFS), plus a live **operating-point fill** under the curve up to

src/editor/meter.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Per-channel level + gain-reduction meters, each with its own latching ceiling/over lamp.
 //!
 //! Each channel is a `|L | GR | R|` cluster (output level left/right, mono gain reduction in the

src/editor/mod.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! egui editor: assembly + control layout.
 //!
 //! The aggregator. Builds the editor window and lays out the heading, the meter panel

src/editor/plot.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Rolling input/output/gain-reduction plot with per-channel tabs and a flow-speed selector.
 //!
 //! Histories for all four channels run continuously (cheap), so switching tabs shows that

src/lib.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 use nih_plug::prelude::*;
 use std::sync::Arc;
 

src/meters.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Lock-free meter state shared from the audio thread to the editor.
 //!
 //! Two feeds, both written by `process()` (single producer) and read by the editor (single

src/params.rs

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: Apache-2.0
 //! Plugin parameters and their layout.
 //!
 //! Holds the global controls plus four `CompressorParams` blocks (low/mid/high + the 'All'