feat(v3): DOM-virtualize the Songs grid (#636 item 3 stage 2) (#643)

The v3 Songs grid appended every scrolled page and never released nodes,
so card-node count grew unbounded with scroll depth (24 → 624 → 2001 for a
2000-song library). Replace it with a windowed/recycled render: only the
visible window (± overscan) is in the DOM while a #v3-songs-gridsizer
element sized to ceil(total/cols)*rowH gives the scrollbar full-library
geometry; #v3-songs-grid is absolutely positioned to the first visible row.

- state.songs is a sparse, absolutely-indexed store filled a page at a time
  by ensureWindow(): the stage-1 keyset cursor for contiguous forward scroll
  (O(page)), OFFSET page= for jumps/restore/non-keyset providers. _loadPage
  shares an in-flight promise per page and an epoch guard discards a stale
  fetch that lands after a reset.
- A–Z rail seeks directly via sort_letters cumulative counts (O(1), no
  page-through); bounded scan fallback for legacy providers without it.
- Snapshot/restore is now scrollTop-based (geometry is stable). Select mode,
  accuracy badges, ⋮ menu, plugin card actions, and tree/folder coexistence
  survive cards recycling; renderWindow re-renders when select mode toggles.
- Plugins get window.v3Songs.visibleCards() + a v3:library-window-rendered
  event instead of assuming all cards are present (highway-stutter lesson).

Verified in a browser against a seeded 2001-song library: DOM bounded to
~60 nodes while the count reads "2001 songs", rail jump lands on the target
row, selection survives recycling, scroll-restore exact. Codex-reviewed
(3 findings fixed: stale-fetch epoch guard, await-in-flight page promise,
select-mode resync on cached re-entry).

Frontend-only. Tests: tests/browser/v3-grid-virtualization.spec.ts pins the
bounded-DOM invariant + direct rail jump; tests/js/v3_az_rail.test.js and
v3_songs_scroll.test.js updated to the new wiring.

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

tests/js/v3_songs_scroll.test.js

@@ -36,13 +36,15 @@ function makeStore() {
     };
 }
 
-function saveSnapshot(storage, state, scrollTop, page, loadedCount) {
+// Mirror of static/v3/songs.js _saveLibraryScrollSnapshot. Under the windowed
+// grid (#636 item 3 stage 2) geometry is stable, so the snapshot is just
+// {hash, scrollTop, view} — no page/loadedCount depth bookkeeping (restore sets
+// scrollTop and re-renders the window that maps to it).
+function saveSnapshot(storage, state, scrollTop) {
     const snap = {
         hash: buildLibraryStateHash(state),
         scrollTop,
         view: state.view,
-        page,
-        loadedCount,
     };
     storage.setItem(SCROLL_STATE_KEY, JSON.stringify(snap));
 }
@@ -88,19 +90,21 @@ test('buildLibraryStateHash is stable for equivalent filter arrays', () => {
     assert.strictEqual(buildLibraryStateHash(s1), buildLibraryStateHash(s2));
 });
 
-test('snapshot stores scrollTop and page', () => {
+test('snapshot stores scrollTop + view + hash (geometry-stable restore)', () => {
     const storage = makeStore();
-    saveSnapshot(storage, baseState, 1840, 3, 96);
+    saveSnapshot(storage, baseState, 1840);
     const snap = readSnapshot(storage);
     assert.strictEqual(snap.scrollTop, 1840);
-    assert.strictEqual(snap.page, 3);
-    assert.strictEqual(snap.loadedCount, 96);
+    assert.strictEqual(snap.view, 'grid');
     assert.strictEqual(snap.hash, buildLibraryStateHash(baseState));
+    // Page-depth bookkeeping is gone — the windowed grid restores from scrollTop.
+    assert.strictEqual(snap.page, undefined);
+    assert.strictEqual(snap.loadedCount, undefined);
 });
 
 test('stale snapshot is detected when filters change', () => {
     const storage = makeStore();
-    saveSnapshot(storage, baseState, 500, 1, 48);
+    saveSnapshot(storage, baseState, 500);
     const snap = readSnapshot(storage);
     const changed = buildLibraryStateHash({ ...baseState, q: 'beatles' });
     assert.notStrictEqual(snap.hash, changed);