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

Cargo.toml

@@ -8,7 +8,7 @@ description = "A VST3/CLAP multiband compressor/limiter with a custom gain-curve
 # VST3 is on by default in nih_plug, and its bindings (vst3-sys) are GPLv3. Any
 # distributed VST3 binary built with them must therefore be GPL-3.0-or-later
 # (or covered by a commercial Steinberg licence). CLAP carries no such requirement.
-license = "GPL-3.0-or-later"
+license = "Apache-2.0"
 
 [workspace]
 members = ["xtask"]