refactor(decouple): move coerceOpenArtistRefs to lib/api (off artist feature)

First step of decoupling the audio core from feature imports. coerceOpenArtist
Refs is a pure Subsonic-response normalizer (one-object-vs-array quirk) that was
mis-membered in features/artist but consumed by the audio core (songToTrack,
trackArtistRefs) and others. Relocate it beside its SubsonicOpenArtistRef type in
lib/api/; drop the artist-barrel re-export; repoint the 4 barrel consumers.

Removes the @/features/artist runtime edge from the audio core (utils/playback)
entirely. tsc 0, lint 0/0, suite 319/2353 green.

(composerBrowseSessionStore's ALL_SENTINEL import from @/features/artist remains
— that's a non-audio global browse store, separate from this decouple.)

src/lib/api/openArtistRefs.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from 'vitest';
+import { coerceOpenArtistRefs } from '@/lib/api/openArtistRefs';
+
+describe('coerceOpenArtistRefs', () => {
+  it('returns an empty array for nullish input', () => {
+    expect(coerceOpenArtistRefs(undefined)).toEqual([]);
+    expect(coerceOpenArtistRefs(null)).toEqual([]);
+  });
+
+  it('passes through arrays', () => {
+    const refs = [{ id: 'a1', name: 'One' }, { id: 'a2', name: 'Two' }];
+    expect(coerceOpenArtistRefs(refs)).toBe(refs);
+  });
+
+  it('wraps a single ref object from Subsonic JSON', () => {
+    const ref = { id: 'a1', name: 'Solo' };
+    expect(coerceOpenArtistRefs(ref)).toEqual([ref]);
+  });
+});