fix(music): address MiniMax review feedback

Move song generation into a first-class music domain and focused CLI, list ready-to-use MLX checkpoints, and remove unrelated converter, packaging, parity-test, and attribution-file surface requested in review.

Verified with 1738 passing repository tests, strict docs and package builds, and finite stereo generation from BF16, MXFP8, and MXFP4 checkpoints.

Co-Authored-By: OpenAI Codex <noreply@openai.com>