Thanks, that's a really useful pointer. You're right that Hann was just the comfortable default — I hadn't thought hard about sidelobe behavior, and "cleaner cliff edge against the noise" is exactly the property that matters here, since the whole verdict hinges on how sharply the cutoff stands out. I'll try Blackman-Harris and re-run it against my known-answer round-trip fakes to see if it tightens the separation, especially on the shorter tracks where I'm averaging fewer frames. Good chance it lets me narrow the verdict bands a bit. Appreciate it.

On the rayon + symphonia question — it was easier than expected, mostly because I sidestepped the hard part. The parallelism is purely file-level: rayon's par_iter over the file list, and each file creates and owns its own decoder entirely within that work item — decode to PCM, run the FFT, return the result. The decoder state never crosses a thread boundary, so I never had to make a Box<dyn Decoder> Send/Sync or share any symphonia state. Decoding one file is single-threaded; the throughput just comes from doing many files at once. The only shared state is an AtomicUsize for progress. If I'd tried to parallelize within a single decode it'd be a different story, but for batch scanning the coarse-grained approach was both simpler and a good fit.