v0.57.13: Real oud voice for Maqam playback

What's fixed

• Maqam.play() and .render() no longer crash. Both default to synth="oud", but the Synth enum had no oud voice, so playing any maqam — Maqam.get("rast").play() or pytheory maqam rast --play — raised KeyError: 'OUD'. There is now a real Synth.OUD: doubled Karplus-Strong courses over a deep bowl resonance with a dark gut-string rolloff, so the advertised default voice actually exists.
• New oud instrument preset. score.part("melody", instrument="oud") works like the other plucked strings, and the synth catalog docs gained an Oud Synth section with an audio demo.

Full Changelog: v0.57.12...v0.57.13