add vst3 subcategories

[erikgrahn13]

What changed

• add singularity_configure_vst3(target SUBCATEGORIES ...) for format-specific plug-in classification
• derive the required Fx or Instrument category from the plug-in class and append one or more configured subcategories
• validate malformed, repeated, missing-target, and oversized configuration
• document the API and configure ExampleEffect as Fx|Tools

Why

VST3-specific metadata should remain at the adapter boundary instead of expanding the format-neutral singularity_create_plugin API or leaking VST3 concepts into framework interfaces.

User impact

Plug-in authors can now declare VST3 classifications such as Dynamics, Tools, or Synth;Sampler while Singularity continues to determine whether the plug-in is an effect or instrument from PLUGIN_CLASS::isInstrument.

Validation

• configured the Debug Ninja build with examples enabled
• built ExampleEffect_VST3 and verified factory metadata reports Fx|Tools
• built unconfigured ExampleInstrument_VST3 and verified the default remains Instrument
• both bundles passed all 47 Steinberg validator tests
• git diff --check