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>

python-backend/musicmouse/__main__.py

@@ -32,7 +32,7 @@ from musicmouse.devices.mouse import MusicMouseDevice
 from musicmouse.devices.null_transport import NullTransport
 from musicmouse.devices.player import Player, VlcPlayer
 from musicmouse.devices.serial_link import SerialLink
-from musicmouse.library import MusicLibrary
+from musicmouse.library import MusicLibrary, default_worker_count
 from musicmouse.library.analysis import build_analyzer
 from musicmouse.reactions import register_all
 from musicmouse.services.base import Service
@@ -283,11 +283,16 @@ def _build_player(bus: EventBus, general: GeneralConfig, *, clock: RealClock) ->
 
 async def build_library(config: Config) -> MusicLibrary:
     library_config = config.general.library
+    workers = library_config.analysis_workers
     return await MusicLibrary.build(
         library_config.root,
         library_config.cache,
         frozenset(config.general.audio_extensions),
         analyzer=build_analyzer(),
+        # Unset in the config means "use the machine": a first pass over an unanalyzed
+        # library is hours of DSP, and there is no reason for a desktop to do it one
+        # core at a time. `MusicLibrary` itself defaults to 1 - see its docstring.
+        analysis_workers=default_worker_count() if workers is None else workers,
         figure_kinds=config.figure_kinds,
     )