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-tauri/crates/psysonic-audio/src/engine.rs

@@ -6,7 +6,7 @@ use std::time::{Duration, Instant};
 use rodio::Player;
 use tauri::{AppHandle, Manager};
 
-use super::state::{ChainedInfo, PreloadedTrack};
+use super::state::{ChainedInfo, PreloadedTrack, StreamCompletedSpill};
 
 /// Reply channel handed back to the audio-stream thread once a re-open finishes.
 pub type StreamReopenReply = std::sync::mpsc::SyncSender<Arc<rodio::MixerDeviceSink>>;
@@ -36,11 +36,17 @@ pub struct AudioEngine {
     /// Last fully downloaded manual-stream track bytes (same playback identity),
     /// used to recover seek/replay without waiting for network again.
     pub(crate) stream_completed_cache: Arc<Mutex<Option<PreloadedTrack>>>,
+    /// On-disk spill for completed ranged streams above `TRACK_STREAM_PROMOTE_MAX_BYTES`.
+    pub(crate) stream_completed_spill: Arc<Mutex<Option<StreamCompletedSpill>>>,
     /// True when the currently playing source supports seeking (in-memory bytes
     /// or `RangedHttpSource`); false for the legacy non-seekable streaming
     /// fallback (`AudioStreamReader`). `audio_seek` rejects with a "not
     /// seekable" error when false so the frontend restart-fallback can engage.
     pub(crate) current_is_seekable: Arc<AtomicBool>,
+    /// HTTP stream paths (`RangedHttpSource`, legacy `AudioStreamReader`): false
+    /// until `TRACK_STREAM_PLAY_START_BYTES` are buffered (or download ends).
+    /// Bytes / local file / radio keep true.
+    pub(crate) stream_playback_armed: Arc<AtomicBool>,
     pub crossfade_enabled: Arc<AtomicBool>,
     pub crossfade_secs: Arc<AtomicU32>,
     pub fading_out_sink: Arc<Mutex<Option<Arc<Player>>>>,
@@ -357,7 +363,9 @@ pub fn create_engine() -> (AudioEngine, std::thread::JoinHandle<()>) {
         eq_pre_gain: Arc::new(AtomicU32::new(0f32.to_bits())),
         preloaded: Arc::new(Mutex::new(None)),
         stream_completed_cache: Arc::new(Mutex::new(None)),
+        stream_completed_spill: Arc::new(Mutex::new(None)),
         current_is_seekable: Arc::new(AtomicBool::new(true)),
+        stream_playback_armed: Arc::new(AtomicBool::new(true)),
         crossfade_enabled: Arc::new(AtomicBool::new(false)),
         crossfade_secs: Arc::new(AtomicU32::new(3.0f32.to_bits())),
         fading_out_sink: Arc::new(Mutex::new(None)),