Support MusicBrainz release track ID

I still don't totally understand when MusicBrainz uses a track ID and
when it uses a release track ID - they're both displayed as the
"MusicBrainz Track ID" tag in Picard, despite being treated as different
tags by everything else - but supporting both is easy enough.

src/mpd_now_playable/cocoa/persistent_id.py

@@ -8,6 +8,7 @@ from ..song import Song
 # The maximum size for a BLAKE2b "person" value is sixteen bytes, so we need to be concise.
 HASH_PERSON_PREFIX: Final = b"mnp.mac."
 TRACKID_HASH_PERSON: Final = HASH_PERSON_PREFIX + b"mb_tid"
+RELEASETRACKID_HASH_PERSON: Final = HASH_PERSON_PREFIX + b"mb_rtid"
 FILE_HASH_PERSON: Final = HASH_PERSON_PREFIX + b"f"
 
 PERSISTENT_ID_BITS: Final = 64
@@ -20,6 +21,14 @@ def digest_trackid(trackid: UUID) -> bytes:
 	).digest()
 
 
+def digest_releasetrackid(trackid: UUID) -> bytes:
+	return blake2b(
+		trackid.bytes,
+		digest_size=PERSISTENT_ID_BYTES,
+		person=RELEASETRACKID_HASH_PERSON,
+	).digest()
+
+
 def digest_file_uri(file: Path) -> bytes:
 	return blake2b(
 		bytes(file), digest_size=PERSISTENT_ID_BYTES, person=FILE_HASH_PERSON
@@ -34,6 +43,8 @@ def digest_file_uri(file: Path) -> bytes:
 def song_to_persistent_id(song: Song) -> int:
 	if song.musicbrainz_trackid:
 		hashed_id = digest_trackid(song.musicbrainz_trackid)
+	elif song.musicbrainz_releasetrackid:
+		hashed_id = digest_releasetrackid(song.musicbrainz_releasetrackid)
 	else:
 		hashed_id = digest_file_uri(song.file)
 	return int.from_bytes(hashed_id)

src/mpd_now_playable/mpd/listener.py

@@ -21,6 +21,9 @@ def mpd_current_to_song(
 		queue_length=int(status["playlistlength"]),
 		file=Path(current["file"]),
 		musicbrainz_trackid=convert_if_exists(current.get("musicbrainz_trackid"), UUID),
+		musicbrainz_releasetrackid=convert_if_exists(
+			current.get("musicbrainz_releasetrackid"), UUID
+		),
 		title=current.get("title"),
 		artist=current.get("artist"),
 		album=current.get("album"),

src/mpd_now_playable/song.py

@@ -19,6 +19,7 @@ class Song:
 	queue_length: int
 	file: Path
 	musicbrainz_trackid: UUID | None
+	musicbrainz_releasetrackid: UUID | None
 	title: str | None
 	artist: str | None
 	composer: str | None