AbstractMusic v0.1.5

Added

• Added a stdlib-only elevenlabs remote backend scoped to ElevenLabs Music endpoints, with
  composition-plan request support and AbstractCore plugin registration.
• Added an explicit stable-audio-3 local backend for stabilityai/stable-audio-3-small-music
  using AbstractMusic-owned runtime code and Hugging Face weights/configs only.

Changed

• Added a provider-neutral MusicCompositionPlan request path so host text planners can hand
  structured music intent to compatible backends without coupling planning to transport code.
• Kept Stable Audio 3 behind its own optional extra and excluded upstream Stable Audio runtime
  packages, Flash-Attn, audio codec libraries, UI, training, LoRA, and local-checkout paths from
  that minimal dependency profile.