v0.12.1: stop asking where the music is when config.json already says

Restarting on the machine that mounts the drive at /media/trav/muzak opened
"LinTunes can't find your music folder", offering ~/Music. The library's only
music_folder is the other machine's mount point (/run/media/trav/tummult/...)
because this library predates 0.10 and has no music_folder_rel yet — but
config.json has held the right path all along under music_root, which
`--music-root … --save-config` writes and the importer stores as
library.music_folder. resolve() only looked for music_folder_override, a key
that exists solely because the prompt writes it, so a library imported the
normal way could never satisfy the check that decides whether to prompt.

music_root is now a machine-local candidate alongside music_folder_override,
consulted in the same last-resort position and subject to the same must-exist
rule, so a stale one still reports "missing" rather than resolving to a phantom
tree. Read-only: nothing writes it back, and the newer key still wins when both
are present.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y9ZEFi4qNJ39FMiBtiAxy2

tests/test_round40.py

@@ -241,6 +241,46 @@ class TestMusicFolderResolution:
         assert state.path == elsewhere
         assert state.source == "machine"
 
+    def test_the_pre_010_music_root_key_counts_as_an_override(
+            self, tmp_path, isolated_config):
+        """`--music-root … --save-config` has always written this machine's
+        media folder to config.json as `music_root`. Every library imported that
+        way already knows where its music is here, so a machine that mounts the
+        drive at a different path must not be asked to go find it."""
+        from lintunes.config import save_config
+        media = tmp_path / "media"
+        media.mkdir()
+        save_config({"music_root": str(media)})
+
+        # The shared value is another machine's mount point, unreachable here.
+        gone = Library(music_folder="/run/media/trav/tummult/music/iTunes Media")
+        state = music_folder.resolve(gone, tmp_path)
+
+        assert state.path == media
+        assert state.source == "machine"
+
+    def test_the_newer_override_key_still_wins(self, tmp_path, isolated_config):
+        from lintunes.config import save_config
+        chosen, imported = tmp_path / "chosen", tmp_path / "imported"
+        chosen.mkdir()
+        imported.mkdir()
+        save_config({"music_folder_override": str(chosen),
+                     "music_root": str(imported)})
+
+        state = music_folder.resolve(Library(music_folder="/nope"), tmp_path)
+
+        assert state.path == chosen
+
+    def test_a_stale_music_root_does_not_resolve(self, tmp_path, isolated_config):
+        """It's still a candidate, not an answer — the folder has to be there."""
+        from lintunes.config import save_config
+        save_config({"music_root": str(tmp_path / "unplugged")})
+
+        state = music_folder.resolve(Library(music_folder="/nope"), tmp_path)
+
+        assert state.path is None
+        assert state.source == "missing"
+
     def test_unset_and_missing_are_different_conversations(self, tmp_path,
                                                            isolated_config):
         assert music_folder.resolve(Library(), tmp_path).source == "unset"