v0.15.0: ask the song what it is

Right-click → Identify Track… fingerprints the file with Chromaprint's
fpcalc and asks AcoustID who it is, then proposes the tags it found.

The year is the point. A 60s song kept getting stamped with the year its
CD reissue came out, which sorts the library wrong, so every candidate
carries the recording's *original* year — the earliest release across all
of its release groups — including the candidate that proposes a later
greatest-hits as the album. Ranking separately prefers a plain studio
Album over EP/Single over anything wearing a Compilation or Live
secondary type, so the default proposal is the real album as well.

Nothing is written until accepted: the dialog shows each current value
beside an editable proposed one, and a row starts checked only where the
proposal actually differs from what the file already has, so an untouched
field can never quietly blank a tag. Applying goes through
edit_track_fields, which is what buys the tag write, the abort-if-it-fails
rule, artist/album file relocation and one undo step without re-deriving
any of them.

A selection is a queue reviewed one track at a time; a failure is a
status-bar message that moves on rather than ending the batch. fpcalc is
detected at runtime like ffmpeg and the AcoustID key lives in preferences
(so it syncs, and stays out of git) — the round adds no new dependency.

Also fixes a pre-existing round 40 test that used the real tummult mount
point as its stand-in for an unreachable path, so it failed whenever that
drive was plugged in.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NsiFHdyVg1UhBSxDJfTNRm

lintunes/preferences.py

@@ -28,6 +28,11 @@ DEFAULTS = {
         "session_key": "",
         "scrobble_enabled": False,
     },
+    # Track identification (Identify Track…). The application key is the
+    # user's own, registered free at acoustid.org/new-application.
+    "acoustid": {
+        "api_key": "",
+    },
 }
 
 
@@ -75,6 +80,15 @@ class Preferences(QObject):
     def lastfm(self) -> dict:
         return self._data["lastfm"]
 
+    def update_acoustid(self, **fields):
+        self._data["acoustid"].update(fields)
+        self.save()
+        self.changed.emit()
+
+    @property
+    def acoustid(self) -> dict:
+        return self._data["acoustid"]
+
     def save(self):
         self._path.parent.mkdir(parents=True, exist_ok=True)
         tmp = self._path.with_suffix(".json.tmp")