v3 library: artist sort orders titles within an artist (tree-view feel) (#720)

* v3 library: artist sort orders titles within an artist (tree-view feel)

Tester report: "the list is set up by artist, but the cards are
alphabetical(-ish random)". Real: the tree orders artist -> album -> title,
while the grid's artist sort ordered within an artist by RAW FILENAME —
community-pack filename noise, so an artist's cards looked shuffled.

- artist / artist-desc gain a title secondary (direction baked per entry so
  the legacy `dir=desc` append can't land on the title term; titles stay
  A->Z under Z->A artists).
- The two-term (value, filename) keyset cursor can't seek a three-term
  order, so artist sorts leave _KEYSET_SORTS and page by OFFSET — measured
  trivial at real library sizes; title/recent keep their keyset. Restore
  via a composite sort-key column if 50k-song libraries ever hurt.
- The tree view says "List view groups by artist — the selected sort
  applies to the card grid" when a non-artist sort is active, instead of
  silently ignoring the picker.
- Keyset proof-tests repinned to the title sort (same property, a sort
  that still keysets); 2 new tests pin the title-within-artist order and
  the OFFSET pagination's no-skip/no-dupe across pages.

Full-suite failure set identical to the same-main baseline.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nm7tHs1Yvjjtnnu4nzJgdN

* v3 library: honor legacy sort=artist&dir=desc (fold dir into effective sort)

Codex/review follow-up to the title-within-artist change: the new artist
ORDER BY bakes in `ASC` (for the title secondary), so the global `dir=desc`
append is suppressed and `sort=artist&dir=desc` silently returned A->Z
instead of Z->A — a regression on the legacy /api/library dir contract.

Fold `dir=desc` into the canonical sort key BEFORE the sort_map lookup via
the existing _effective_keyset_sort helper (same fold the cursor side already
does), so the ORDER BY is built from the effective sort. Only artist/title
fold (they have `-desc` twins); title/recent/tuning/year/mastery are
unaffected — verified by the keyset/filter suites.

New test pins that legacy `sort=artist&dir=desc` matches the explicit
`artist-desc` ordering (Z->A artists, A->Z titles within each).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

tests/test_library_keyset.py

@@ -73,7 +73,10 @@ def _walk_offset(client, sort, size, total):
     return seen
 
 
-@pytest.mark.parametrize("sort", ["artist", "artist-desc", "title", "title-desc", "recent"])
+# artist/artist-desc left out: their ORDER BY carries a title secondary
+# (grid cards read alphabetically within an artist, like the tree), which
+# the two-term cursor cannot seek — they page by OFFSET (covered below).
+@pytest.mark.parametrize("sort", ["title", "title-desc", "recent"])
 def test_keyset_matches_offset_exactly(client, server_mod, sort):
     _seed(server_mod, 25)
     offset_order = _walk_offset(client, sort, 7, 25)
@@ -87,15 +90,17 @@ def test_stable_tiebreak_on_equal_keys(client, server_mod):
     # 25 songs, all the SAME artist → the artist sort is decided entirely by the
     # filename tiebreak. Both pagers must still cover all 25 with no dupe.
     _seed(server_mod, 25, shared_artist=True)
-    keyset_order = _walk_keyset(client, "artist", 6, 25)
+    keyset_order = _walk_offset(client, "artist", 6, 25)
     assert len(keyset_order) == 25 and len(set(keyset_order)) == 25
     assert keyset_order == sorted(keyset_order)      # tiebreak is filename ASC
 
 
 def test_first_page_has_cursor_and_no_after_is_offset(client, server_mod):
     _seed(server_mod, 5)
+    body = client.get("/api/library", params={"sort": "title", "size": 2}).json()
+    assert body["next_cursor"]                       # keyset sort: cursor offered
     body = client.get("/api/library", params={"sort": "artist", "size": 2}).json()
-    assert body["next_cursor"]                       # cursor offered
+    assert body["next_cursor"] is None               # artist sorts page by OFFSET
     assert [s["filename"] for s in body["songs"]] == ["song00.archive", "song01.archive"]
 
 
@@ -111,7 +116,7 @@ def test_legacy_dir_desc_keysets_correctly(client, server_mod):
     _seed(server_mod, 20)
     offset_order, page = [], 0
     while True:
-        body = client.get("/api/library", params={"sort": "artist", "dir": "desc", "size": 6, "page": page}).json()
+        body = client.get("/api/library", params={"sort": "title", "dir": "desc", "size": 6, "page": page}).json()
         if not body["songs"]:
             break
         offset_order.extend(s["filename"] for s in body["songs"])
@@ -119,7 +124,7 @@ def test_legacy_dir_desc_keysets_correctly(client, server_mod):
     keyset, cursor, guard = [], "", 0
     while len(keyset) < 20 and guard < 25:
         guard += 1
-        params = {"sort": "artist", "dir": "desc", "size": 6}
+        params = {"sort": "title", "dir": "desc", "size": 6}
         if cursor:
             params["after"] = cursor
         body = client.get("/api/library", params=params).json()
@@ -131,7 +136,7 @@ def test_legacy_dir_desc_keysets_correctly(client, server_mod):
     assert len(set(keyset)) == 20
 
 
-@pytest.mark.parametrize("sort", ["artist", "artist-desc", "recent"])
+@pytest.mark.parametrize("sort", ["recent"])
 def test_keyset_handles_null_sort_keys(client, server_mod, sort):
     # NULL artist/mtime (corrupt/legacy rows past put()'s '' defaults) sort
     # first in ASC / last in DESC; keyset must cover them exactly like OFFSET.