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

Apache-2.0 was the wrong declaration for two independent reasons.

Enabling the `vst3` feature links vst3-sys, which is GPLv3-or-later by its
own authors' choice — they describe their port as a derivative of
Steinberg's SDK that "was not made in isolation", so the obligation holds
regardless of any question about Steinberg's own rights. Gating the
feature kept the default build clean, but plugins built from this template
are intended to ship VST3, so GPL is where they land anyway. Declaring it
up front is more honest than deferring the decision to every clone.

Separately, parts of src/lib.rs derive from Robbert van der Helm's
NIH-plug cookiecutter template and its `gain` example, both ISC. ISC's one
condition is that its copyright and permission notice travel with all
copies, and this repo carried neither — a compliance gap that existed
regardless of the GPL question. ISC is GPL-compatible, so the notice is
retained in a new NOTICE file rather than superseded.

The `vst3` feature stays opt-in, but its rationale changes: it is no
longer about licence compliance, only about not pulling a sizeable
dependency into a CLAP-only build. The Cargo.toml comment said otherwise
and would have misled the next reader.

Also corrects the README's claim that you must "accept Steinberg's VST3
SDK licence". No SDK source is vendored or redistributed here, so there is
nothing to accept at build time. The real constraint is vst3-sys's GPLv3,
plus Steinberg's separate trademark in "VST" — a different regime from
copyright 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

NOTICE

@@ -0,0 +1,46 @@
+nih-plug-template
+Copyright 2026 Mikkeli Matlock
+
+This product is licensed under the GNU General Public License v3.0 or later.
+See LICENSE for the full terms.
+
+--------------------------------------------------------------------------------
+Third-party material
+--------------------------------------------------------------------------------
+
+Portions of src/lib.rs derive from the NIH-plug cookiecutter template and from
+NIH-plug's `gain` example plugin, both by Robbert van der Helm:
+
+    https://github.com/robbert-vdh/nih-plug-template
+    https://github.com/robbert-vdh/nih-plug
+
+Those portions are licensed under the ISC License:
+
+    ISC License
+
+    Copyright (c) 2022 Robbert van der Helm
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+    MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+    SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+    OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+    CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+ISC is GPL-compatible, so this notice is retained rather than superseded.
+
+--------------------------------------------------------------------------------
+Note on the `vst3` feature
+--------------------------------------------------------------------------------
+
+Building with `--features vst3` links `vst3-sys`, which is licensed GPLv3 or
+later. This is the reason a distributed VST3 build must be GPL-licensed. No
+Steinberg SDK source is vendored or redistributed by this project; "VST" is a
+trademark of Steinberg Media Technologies GmbH, and using it to describe a
+distributed product is governed by Steinberg's trademark terms, separately from
+any copyright licence above.