v0.20.3: one artist spelled two ways

The sync was fine. Everything it put on the Rabbit was there and correct --
2,432 media files, 208 covers, a manifest whose every field type-checks
against the parser -- and andTunes still answered "No library / Couldn't read
library.json". Fifteen tracks took all 2,439 songs down with them.

Library.group() keyed albumsByKey case-folded and artistsByName raw-case, and
only ever built the Artist inside `if (album == null)`. So the second track of
an album whose artist is spelled differently found the album already there,
skipped the block that would have made the artist, and dereferenced the null
that came back. Six artists in trav's library are spelled two ways: RJD2/Rjd2,
Toro Y Moi/Toro y Moi, FatBoy Slim/Fatboy Slim, LOVING/Loving, Land Of The
Loops/Land of the Loops, Salami Rose Joe Louis/salami rose joe louis. It had
worked until the 12th because that sync was the first to carry both spellings
of one of those albums.

Both maps fold case now, and the artist is fetched-or-made before the album
block and held, so no lookup left in group() can come back null. Artist gains
a key the way Album always had one, and ListActivity navigates by it --
ALBUM_ROW already passed album.key, so artists just stopped being the
exception.

The device filesystem is case-insensitive, so those two spellings are one
folder there. plan_andtunes_sync compared exact strings and saw every such
file as stale *and* missing, deleting and re-copying it over MTP on every
sync forever; the diff and the collision rule both fold now. plan.stale still
carries the device's own spelling, since that is what _delete_stale unlinks by.

Verified on the Rabbit: 2,439 songs listed, RJD2 one row of 12 songs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SKXUgsBBwe3qaHEjeV8ubP

CLAUDE.md

@@ -338,6 +338,15 @@ persistence) → GUI (Qt widgets that read the manager and connect to its signal
   `device_sync` (so it rides the Syncthing share and both machines agree);
   keeping it off the `Playlist` keeps it out of the conflict-merge machinery,
   and **unticking is the only way a playlist comes off the device**.
+  **The device's filesystem is case-insensitive**, so "RJD2" and "Rjd2" are one
+  folder there and one artist here: since Round 54 the app's `Library.group()`
+  keys *both* `albumsByKey` and `artistsByName` case-folded (and fetches the
+  artist before the album block, holding it — the raw-case lookup afterwards is
+  what NPE'd on the second track of a two-spelling album and blanked the whole
+  library), and `plan_andtunes_sync` folds both the collision rule and the
+  device diff, which had been re-copying every such file over MTP every sync.
+  `plan.stale` still carries the device's own spelling — that is what
+  `_delete_stale` unlinks by.
   Since Round 50 the app exists: `andtunes/app/` is plain Java against the
   Android framework, built by `andtunes/build.py` (aapt2 → javac → R8 →
   zipalign → apksigner) — **no Gradle, no Kotlin**, because platform 33 +