fix: cut long names from the middle, not the end

The shortening fitted the path and destroyed its meaning. Lidarr writes
"Artist - Album - 07 - Flamethrower.mp3" inside a directory already named for
that artist and album, so a long album title occurs three times in one path and
everything that distinguishes one track from another sits at the very end.
Cutting from the end removed precisely that:

  King Gizzard & the Lizard Wizard - PetroDragonic Apocalypse; or, Dawn of Eter~c526.mp3

All seven tracks on that record reduced to the same string bar the hash. The
path fitted; the result was seven files nobody could tell apart on the device,
which is a worse outcome than the failure it replaced.

Cut from the middle instead, giving two thirds of the remaining room to the
tail because the head is generally a restatement of the directory the file
already sits in:

  King Gizzard & the Lizard~c526~ginning of Merciless Damnation - 07 - Flamethrower.mp3

The eight real paths that prompted this are now regression tests: every track
on that album keeps its number and title, all seven names stay distinct, and
The Beatles' "The Long One" -- whose length is the title itself rather than a
repeated album name -- keeps both ends.

tests/test_music_mirror.py

@@ -705,3 +705,62 @@ def test_a_path_that_cannot_be_made_to_fit_is_reported(tmp_path, make_flac, capl
         run(source, mirror, "--fat32-safe", "--max-path", "80")
 
     assert "too" in caplog.text and "nested" in caplog.text
+
+
+# Lidarr writes "Artist - Album - NN - Title.mp3" inside a directory already
+# named for that artist and album, so a long album title appears three times in
+# one path. These are real paths from a real library.
+GIZZARD_ALBUM = (
+    "PetroDragonic Apocalypse; or, Dawn of Eternal Night - An Annihilation of"
+    " Planet Earth and the Beginning of Merciless Damnation"
+)
+GIZZARD_TRACKS = (
+    "01 - Motor Spirit",
+    "02 - Supercell",
+    "03 - Converge",
+    "04 - Witchcraft",
+    "05 - Gila Monster",
+    "06 - Dragon",
+    "07 - Flamethrower",
+)
+
+
+def gizzard_path(track):
+    return Path(
+        "King Gizzard & the Lizard Wizard",
+        f"{GIZZARD_ALBUM} (2023)",
+        f"King Gizzard & the Lizard Wizard - {GIZZARD_ALBUM} - {track}.mp3",
+    )
+
+
+def test_shortening_keeps_the_part_that_tells_tracks_apart():
+    """Cutting from the end discards the track number and title, which is all
+    that distinguishes one track on the record from another."""
+    for track in GIZZARD_TRACKS:
+        fitted = music_mirror.fit_path(gizzard_path(track), 253)
+        assert len(str(fitted)) <= 253
+        assert fitted.name.endswith(f"{track}.mp3"), fitted.name
+
+
+def test_every_track_on_a_long_album_keeps_a_distinct_name():
+    fitted = {music_mirror.fit_path(gizzard_path(t), 253).name for t in GIZZARD_TRACKS}
+
+    assert len(fitted) == len(GIZZARD_TRACKS)
+
+
+def test_a_long_title_keeps_both_ends():
+    """The Beatles' 'The Long One' is one track whose title is the long part."""
+    path = Path(
+        "The Beatles",
+        "Abbey Road (1969)",
+        "Digital Media 03",
+        "The Beatles - Abbey Road - 09 - The Long One - You Never Give Me Your Money"
+        " + Sun King + Mean Mr Mustard + Her Majesty + Polythene Pam + She Came In"
+        " Through the Bathroom Window+ Golden Slumbers + Carry That Weight + The End.mp3",
+    )
+
+    fitted = music_mirror.fit_path(path, 253)
+
+    assert len(str(fitted)) <= 253
+    assert fitted.name.startswith("The Beatles - Abbey Road - 09 - The Long One")
+    assert fitted.name.endswith("The End.mp3")