Fix AUv2 main channel capability reporting

[satoshi-assistant]

Summary

• report AUv2 channel capabilities from main input and output ports only
• keep the existing zero-channel fallback when no main port exists

Root cause

SupportedNumChannels() combined channel counts from every bus. A differently sized auxiliary bus was therefore advertised as a supported main input/output configuration, even though ValidFormat() correctly rejected that channel count for the main bus.

Validation

• built a multi-bus AUv2 plugin with a stereo main input, mono auxiliary input, and stereo main output
• confirmed auval reports only [2, 2] and completes successfully
• git diff --check