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notice is retained in NOTICE rather than superseded.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