feat(web): crossfade between tracks (Tier C11)

Operator-tunable 0-12s crossfade in Settings → Playback. Default 0
(off). Most albums sound best at 0 — gapless masters, classical, and
live recordings all suffer noticeable crossfades.

Implementation: pure-function position-derived volume scalar.
`deriveFadeScalar(position, duration, crossfadeSec)` ramps from 0 to
1 over the leading X seconds, 1 to 0 over the trailing X seconds,
and stays at 1 in between. Tracks shorter than 2X don't fade.

The layout's audio-volume effect now multiplies player.volume by
the fade scalar — no timers, no AudioContext, no element swapping.
Re-renders at the ~4Hz `timeupdate` cadence give 16 discrete steps
over a 4s fade, audibly close to smooth. Smoothing to per-frame
ramps via rAF is a follow-up if needed.

Setting persists to localStorage as `minstrel.crossfade` (matches
the existing volume-storage convention).

Tests cover the pure derivation (off, too-short track, fade-in,
fade-out) + clamp/persist on setCrossfade + invalid-input recovery
on readStoredCrossfade.

web/src/lib/player/store.test.ts

@@ -591,3 +591,55 @@ describe('queue mutations', () => {
     expect(player.queue.map((x) => x.id)).toEqual(['a']);
   });
 });
+
+describe('crossfade', () => {
+  test('deriveFadeScalar returns 1 when crossfadeSec is 0', async () => {
+    const { deriveFadeScalar } = await import('./store.svelte');
+    expect(deriveFadeScalar(50, 200, 0)).toBe(1);
+    expect(deriveFadeScalar(0, 200, 0)).toBe(1);
+    expect(deriveFadeScalar(200, 200, 0)).toBe(1);
+  });
+
+  test('deriveFadeScalar returns 1 when track is too short to fade twice', async () => {
+    const { deriveFadeScalar } = await import('./store.svelte');
+    // 6s track with 4s crossfade: 6 < 4*2, so no fade.
+    expect(deriveFadeScalar(0, 6, 4)).toBe(1);
+    expect(deriveFadeScalar(3, 6, 4)).toBe(1);
+  });
+
+  test('deriveFadeScalar ramps in over the leading X seconds', async () => {
+    const { deriveFadeScalar } = await import('./store.svelte');
+    // 60s track, 4s crossfade.
+    expect(deriveFadeScalar(0, 60, 4)).toBe(0);
+    expect(deriveFadeScalar(2, 60, 4)).toBe(0.5);
+    expect(deriveFadeScalar(4, 60, 4)).toBe(1);
+  });
+
+  test('deriveFadeScalar ramps out over the trailing X seconds', async () => {
+    const { deriveFadeScalar } = await import('./store.svelte');
+    expect(deriveFadeScalar(56, 60, 4)).toBe(1);
+    expect(deriveFadeScalar(58, 60, 4)).toBe(0.5);
+    expect(deriveFadeScalar(60, 60, 4)).toBe(0);
+  });
+
+  test('setCrossfade clamps to [0, 12] and persists', async () => {
+    const { setCrossfade, player } = await import('./store.svelte');
+    setCrossfade(4);
+    expect(player.crossfadeSec).toBe(4);
+    expect(localStorage.getItem('minstrel.crossfade')).toBe('4');
+    setCrossfade(99);
+    expect(player.crossfadeSec).toBe(12);
+    setCrossfade(-3);
+    expect(player.crossfadeSec).toBe(0);
+  });
+
+  test('readStoredCrossfade survives an invalid stored value', async () => {
+    const { readStoredCrossfade } = await import('./store.svelte');
+    localStorage.setItem('minstrel.crossfade', 'banana');
+    expect(readStoredCrossfade()).toBe(0);
+    localStorage.setItem('minstrel.crossfade', '500');
+    expect(readStoredCrossfade()).toBe(0);
+    localStorage.setItem('minstrel.crossfade', '7');
+    expect(readStoredCrossfade()).toBe(7);
+  });
+});