feat: support configurable lyrics output path for music generate

[caozeal]

Summary

• add --lyrics-out to mmx music generate so callers can choose a lyrics output file or directory, defaulting to the current directory
• save lyrics for direct lyrics input, --lyrics-optimizer, and --output-format url flows
• call the official lyrics_generation endpoint before music generation when --lyrics-optimizer is used so the saved lyrics match the generated song

Test Plan

• bun test
• bun run build
• git diff --check