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>

static/v3/songs.js

@@ -2247,6 +2247,11 @@
     async function loadTree() {
         const host = document.getElementById('v3-songs-tree');
         if (!host) return;
+        // The list view always groups artist -> album (query_artists has no
+        // free sort) — when the picked sort is something else, say so instead
+        // of silently ignoring it ("why does the sort do nothing here?").
+        const _treeSortNote = railSortColumn() === 'artist' ? ''
+            : '<p class="text-xs text-fb-textDim mb-3">List view groups by artist — the selected sort applies to the card grid.</p>';
         // Capture expanded groups BEFORE the "Loading…" wipe below, so a reload
         // (e.g. toggling select mode) restores them instead of collapsing all.
         const openArtists = new Set(
@@ -2266,7 +2271,7 @@
         }
         if (!artists.length) { host.innerHTML = '<p class="text-fb-textDim text-sm">Nothing here.</p>'; return; }
         artists.forEach((a) => (a.albums || []).forEach((al) => (al.songs || []).forEach((s) => { state.songsById[cardKey(s)] = s; })));
-        host.innerHTML = artists.map((a) =>
+        host.innerHTML = _treeSortNote + artists.map((a) =>
             '<details data-artist="' + esc(a.name) + '"' + (openArtists.has(a.name) ? ' open' : '') + ' class="border-b border-fb-border/40"><summary class="cursor-pointer py-2 text-fb-text flex items-center justify-between">' +
             '<span>' + esc(a.name) + '</span><span class="text-xs text-fb-textDim">' + esc(a.song_count) + '</span></summary>' +
             '<div class="pl-3 pb-2 space-y-2">' + (a.albums || []).map((al) =>