chore: relicense to GPL-3.0-or-later and credit the ISC upstream

Commit 057d630 switched this repo from GPL-3.0 to Apache-2.0, which the
build contradicts: src/lib.rs calls nih_export_vst3! unconditionally and
nih_plug is taken with default features, so every build links vst3-sys —
GPLv3-or-later by its own authors' choice. deploy.ps1 then installs that
binary. The declared licence and the artifact's actual terms disagreed,
and the original GPL-3.0 was correct.

Given the intent to distribute non-profit and open-source, GPL costs
nothing here, so this restores it rather than dropping VST3 to keep a
permissive licence.

Also adds the ISC notice owed to Robbert van der Helm: the plugin
scaffolding derives from the NIH-plug cookiecutter template and example
plugins, whose one condition is that the copyright and permission notice
travel with all copies. The README credited the template under Reference
Material, which is courtesy, not compliance. ISC is GPL-compatible, so the
notice is retained in NOTICE rather than superseded.

Corrects the README's claim that Steinberg's SDK licence must be accepted
before distributing. No SDK source is vendored or redistributed, so there
is nothing to accept at build time. The binding constraint is vst3-sys's
GPLv3; Steinberg's trademark in "VST" is a separate regime that no source
licence resolves.

Copyright in the Irene <irene@novoyuuparosk.org> commits is held by the
repository owner (an agent identity), so no third-party agreement was
needed for this relicense.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WHt4DykxXaQnyaJfyS8KCn

src/dsp/biquad.rs

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! Generic second-order IIR biquad, Transposed Direct Form II.
 //!
 //! Coefficient formulas are the RBJ Audio EQ Cookbook

src/dsp/compressor.rs

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! Single full-band feed-forward compressor with look-ahead.
 //!
 //! Design follows Giannoulis, Massberg & Reiss, "Digital Dynamic Range Compressor

src/dsp/crossover.rs

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! 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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! 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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! DSP building blocks for Codename 206.
 //!
 //! The signal chain: a `crossover` filterbank splits into bands, each band (plus the summed

src/dsp/oversampler.rs

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! 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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! 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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! 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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! 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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! egui editor: assembly + control layout.
 //!
 //! The aggregator. Builds the editor window and lays out the heading, the meter panel

src/editor/plot.rs

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! 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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 use nih_plug::prelude::*;
 use std::sync::Arc;
 

src/meters.rs

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! 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

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: Apache-2.0
+// SPDX-License-Identifier: GPL-3.0-or-later
 //! Plugin parameters and their layout.
 //!
 //! Holds the global controls plus four `CompressorParams` blocks (low/mid/high + the 'All'