feat(playback): stream buffering UI, M4A moov-at-end streaming, hot-cache spill (#737)

* feat(playback): stream buffering UI, ranged M4A tail prefetch, demuxer fix

Defer seekbar/progress until HTTP stream is armed for both legacy and
RangedHttpSource; show buffering overlay on cover art. Add MP4 tail
prefetch and Symphonia isomp4 bounded-mdat/moov-at-EOF probing so
moov-at-end M4A can start without reading the full mdat.

* feat(hot-cache): spill large ranged streams to disk for promote

When a ranged HTTP download completes above the 64 MiB RAM promote cap,
write the existing buffer once to app-data stream-spill/ and register it
for hot-cache promote (rename) and replay via fetch_data. Analysis seeds
from the spill file up to the local-file cap (512 MiB).

* fix(ui): stream buffering — grayscale cover and static clock icon

Desaturate player and queue cover art while isPlaybackBuffering; keep a
non-animated clock overlay for visibility without the spinning animation.

* fix(playback): review follow-up — tests, i18n, spill cleanup, changelog

Clippy and test layout fixes; stream spill orphan cleanup on startup;
buffering flag guard in progress handler; bufferingStream in all player
locales; CHANGELOG and contributor credits for stream/M4A work.

* docs: attribute stream buffering and M4A streaming to PR #737

* test(audio): avoid create_engine in stream spill unit test

CI runners have no audio output device; test spill take/consume via
the Mutex slot only, matching install_stream_completed_spill tests.

src/components/playback/PlaybackBufferingOverlay.test.tsx

@@ -0,0 +1,13 @@
+import { describe, expect, it } from 'vitest';
+import { screen } from '@testing-library/react';
+import { renderWithProviders } from '@/test/helpers/renderWithProviders';
+import { PlaybackBufferingOverlay } from './PlaybackBufferingOverlay';
+
+describe('PlaybackBufferingOverlay', () => {
+  it('exposes buffering status for assistive tech', () => {
+    renderWithProviders(<PlaybackBufferingOverlay />);
+    expect(
+      screen.getByRole('status', { name: 'Loading track from server' }),
+    ).toBeInTheDocument();
+  });
+});

src/components/playback/PlaybackBufferingOverlay.tsx

@@ -0,0 +1,17 @@
+import { Clock } from 'lucide-react';
+import { useTranslation } from 'react-i18next';
+
+/** Shown over cover art while an HTTP stream is still opening / buffering. */
+export function PlaybackBufferingOverlay() {
+  const { t } = useTranslation();
+  return (
+    <div
+      className="playback-buffering-overlay"
+      role="status"
+      aria-live="polite"
+      aria-label={t('player.bufferingStream')}
+    >
+      <Clock size={26} strokeWidth={2} className="playback-buffering-overlay__icon" aria-hidden />
+    </div>
+  );
+}