Feature/audio outerproducthmm

[laurenceyoon]

(Apologies for the big PR)

What is changed

• New AudioOuterProductHMM: 2-layer HMM with sound/pause sub-states per score position. Uses 88-bin CQT harmonic mask template matching and spectral-flux-driven transitions. Uses CQTSpectralFluxProcessor feature (89-dim).
• Unified score loading & evaluation refactor: Score loading now uses partitura.load_score() + merge_parts() (supports MusicXML, MEI, etc).
• Simplified debug output: Debug now saves warping path TSV, results JSON, and a single alignment plot.
• Asset updates: Renamed example files to simple_mozart_k265_var1.*, added MEI/match/annotation files for Mozart K265 and Bach BWV 858.

[CarlosCancino-Chacon]

There are a couple minimal changes, but I otherwise we can merge this branch. Some things might need to be unified later, after all methods are there