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/stream/mod.rs

@@ -11,6 +11,7 @@
 
 mod icy;
 mod local_file;
+mod mp4;
 mod radio;
 mod ranged_http;
 mod reader;
@@ -32,7 +33,8 @@ pub(crate) const RADIO_BUF_CAPACITY: usize = 256 * 1024;
 pub(crate) const TRACK_STREAM_MIN_BUF_CAPACITY: usize = 1024 * 1024;
 /// Cap ring buffer growth when content-length is known.
 pub(crate) const TRACK_STREAM_MAX_BUF_CAPACITY: usize = 32 * 1024 * 1024;
-/// Max bytes kept in memory to promote a completed streamed track for fast replay/seek recovery.
+/// Max bytes kept in RAM (`stream_completed_cache`) for fast replay; larger completed
+/// ranged streams are spilled under app-data `stream-spill/` for hot-cache promote.
 pub(crate) const TRACK_STREAM_PROMOTE_MAX_BYTES: usize = 64 * 1024 * 1024;
 /// Hot/offline `psysonic-local://` files are read from disk for waveform/LUFS seeding — not the
 /// same heap pressure as retaining a full HTTP capture. FLAC/DSD tracks often exceed 64 MiB;
@@ -42,7 +44,26 @@ pub(crate) const LOCAL_FILE_PLAYBACK_SEED_MAX_BYTES: usize = 512 * 1024 * 1024;
 pub(crate) const TRACK_STREAM_MAX_RECONNECTS: u32 = 3;
 /// Seconds at stall threshold while paused before hard-disconnect.
 pub(crate) const RADIO_HARD_PAUSE_SECS: u64 = 5;
-/// AudioStreamReader timeout: if no audio bytes arrive for this long → EOF.
+/// Live radio: if no audio bytes arrive for this long → EOF.
 pub(crate) const RADIO_READ_TIMEOUT_SECS: u64 = 15;
+/// On-demand tracks (`track-stream`, `RangedHttpSource`): allow long gaps while a
+/// large file is still downloading (format probe may read/seek ahead of the filler).
+pub(crate) const TRACK_READ_TIMEOUT_SECS: u64 = 120;
+/// HTTP track paths (`AudioStreamReader`, `RangedHttpSource`): minimum linear
+/// download before audible playback and seekbar progress (demux probe may read
+/// far ahead of the play cursor).
+pub(crate) const TRACK_STREAM_PLAY_START_BYTES: u64 = 384 * 1024;
+
+/// Arm deferred playback / progress once enough of the file is buffered.
+pub(crate) fn maybe_arm_stream_playback(downloaded: u64, playback_armed: &std::sync::atomic::AtomicBool) {
+    use std::sync::atomic::Ordering;
+    if !playback_armed.load(Ordering::Relaxed) && downloaded >= TRACK_STREAM_PLAY_START_BYTES {
+        playback_armed.store(true, Ordering::SeqCst);
+        crate::app_deprintln!(
+            "[stream] playback armed after {} KiB buffered",
+            downloaded / 1024
+        );
+    }
+}
 /// Sleep interval when ring buffer is empty (prevents CPU spin).
 pub(crate) const RADIO_YIELD_MS: u64 = 2;