Instrument solos

Add the ability to request a solo for a specific instrument in a generated track.

What it would do:

• Let users specify which instrument takes the solo (e.g. [electric guitar solo], [piano solo], [saxophone solo]).

• Optionally let users indicate where the solo should land in the song (intro, bridge, outro) and roughly how long it should run.

Why it's useful:

• Gives users finer control over song structure and dynamics instead of relying on the model to place instrumental moments on its own.

• Helps create more expressive, performance-style arrangements that stand out.