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.

Claude-Session: https://claude.ai/code/session_01Y9ZEFi4qNJ39FMiBtiAxy2

lintunes/__init__.py

@@ -1,3 +1,3 @@
 """LinTunes — iTunes-style music library manager and player for Linux."""
 
-__version__ = "0.12.0"
+__version__ = "0.12.1"

lintunes/music_folder.py

@@ -10,7 +10,9 @@ single place that decides, in this order:
    machine even when the absolute paths are not.
 2. ``music_folder`` — the legacy absolute value. Libraries written before 0.10,
    and libraries last written by an older LinTunes, only have this one.
-3. The machine-local override in ``~/.config/lintunes/config.json``.
+3. The machine-local override in ``~/.config/lintunes/config.json`` — either
+   ``music_folder_override`` (set by the folder prompt) or the older
+   ``music_root`` that ``--music-root … --save-config`` has always written.
 
 The override is consulted **last, not first**, on purpose. If it won outright, a
 machine that once needed an override would be pinned to it forever and would
@@ -32,6 +34,13 @@ from lintunes.paths import to_absolute
 # Keys in the per-machine config.json (never synced).
 OVERRIDE_KEY = "music_folder_override"
 PROMPT_SEEN_KEY = "music_folder_prompted"
+# The same fact under its pre-0.10 name: `--music-root … --save-config` has
+# always written this machine's media folder here, and the importer stores it as
+# `library.music_folder`. Every library set up that way therefore has the right
+# answer sitting in config.json, and ignoring it meant a machine that mounts the
+# drive somewhere else was asked to locate a folder it had already been told
+# about. Read-only — nothing writes it back, so the newer key stays canonical.
+LEGACY_OVERRIDE_KEY = "music_root"
 
 
 @dataclass(frozen=True)
@@ -73,8 +82,10 @@ def resolve(library, data_dir, config=None) -> Resolution:
         if found is not None:
             return Resolution(found, "legacy", legacy)
 
-    override = config.get(OVERRIDE_KEY, "")
-    if override:
+    for key in (OVERRIDE_KEY, LEGACY_OVERRIDE_KEY):
+        override = config.get(key, "")
+        if not override:
+            continue
         found = _existing_dir(override)
         if found is not None:
             return Resolution(found, "machine", override)