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/lib.rs

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