Analyse tracks in a pool of worker processes

A first pass over an unanalysed library is hours of librosa, and there was no
reason for a desktop to spend them one core at a time. Analysis now runs in a
process pool sized by `general.library.analysis_workers`, defaulting to one per
core bar one (capped at 8) when the key is absent.

Two consequences worth knowing before touching this: whatever an `Analyzer`
returns has to be picklable, and an analyzer that keeps state on its own
instance - a test double counting calls - only behaves as written with
`analysis_workers=1`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/REPO_OVERVIEW.md

@@ -123,3 +123,7 @@ reported at once.
   and size, so **a change to how the scanner derives a title, artist or series is
   invisible until the cache is invalidated** — bump `_INDEX_VERSION` in
   `musicmouse/library/cache.py` when you touch that logic.
+- Track analysis (librosa) runs in a pool of worker *processes* - see
+  `musicmouse/library/workers.py`. Anything an `Analyzer` returns therefore has to be
+  picklable, and an analyzer that records state in its own instance (a test double
+  counting calls) only behaves as written with `analysis_workers=1`.