Load a Song from a Standard MIDI File via the HCodecs library.

This is a lossy, rhythm-free conversion: it exists to get MIDI note data onto the fretless (non-tab) side of the model quickly, not to preserve a MIDI file's original structure. Every NoteOn event becomes its own Quarter-duration Beat (so original note timing/duration is discarded), and every converted Track is placed entirely under MeasureIndex 0 (so songMeasures is left empty — there is no bar structure to derive one from).