menuband: 'Use AC OS MIDI' — share gm_synth.c, audition AC GM voices locally master
Wire the standalone AC GM C-synthesis (fedac/native/src/gm_synth.c) into Menu Band so its GM instruments can use the real AC OS voices instead of the system MIDISynth soundfont, toggled by an About-screen checkbox — the local audition loop for the GM-synthesis campaign (no device/oven needed). - Package.swift: new CGMSynth C target compiling gm_synth.c via SYMLINK (single source of truth) + module map. - MenuBandGMSynth.swift: AVAudioSourceNode poly node, 24-voice preallocated pool, per-voice GMVoice + AR envelope, per-trigger seed (stochastic), gm_program_implemented gate. - AboutWindow: 'Use AC OS MIDI' checkbox (UserDefaults menuband.useACMIDI, default off) → notification → synth.setUseACMIDI. - MenuBandSynth: noteOn/noteOff/setMelodicProgram route to the GM node when enabled + program implemented, else fall back to MIDISynth; drums/radio/ sample paths untouched. swift build clean.