No way to know where a song's sections are without finding them by ear

[thcp]

The problem

StemDeck can tell you where every beat is, but nothing about where the song
goes. To loop the second chorus, or drop into the bridge, or hand a bandmate a
timestamp, you have to find the boundary by ear and drag a marker there
yourself, for every song, every time.

The sections bar already exists and is already editable. What is missing is
anything to put in it.

What makes this harder than it sounds

• Model output is untrusted input. Whatever produces the suggestion is a
  third-party model. Its timestamps, its labels, and its structure all have to
  be validated before they touch a job, and a malformed result has to leave the
  separation exactly as it was.
• It costs real time. Inference is a CPU pass measured in minutes. A user
  who only wanted stems should not silently pay for it, which means it needs to
  be a visible choice rather than an always-on cost.
• The stems are the wrong shape. StemDeck separates six stems; the models
  in this space expect four. Mapping between them without running a second
  separation is a constraint on any solution.
• Labels will be wrong sometimes. Functional names are not ground truth
  even when the boundaries are right, so whatever ships has to present them
  honestly and stay editable rather than claiming more confidence than it has.
• Failure must be non-fatal. This is the last stage of a pipeline that has
  already done the expensive work. Nothing here may cost a user their
  separation, and cancellation has to stay responsive throughout.