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/commands.rs

@@ -15,8 +15,8 @@ use super::engine::{audio_http_client, AudioEngine};
 use super::helpers::*;
 use super::ipc::{maybe_emit_normalization_state, NormalizationStatePayload};
 use super::play_input::{
-    build_source_from_play_input, select_play_input, swap_in_new_sink, url_format_hint,
-    PlayInputContext, SinkSwapInputs,
+    build_source_from_play_input, select_play_input, spawn_legacy_stream_start_when_armed,
+    swap_in_new_sink, url_format_hint, PlayInputContext, SinkSwapInputs,
 };
 use super::preview::preview_clear_for_new_main_playback;
 use super::progress_task::spawn_progress_task;
@@ -97,6 +97,8 @@ pub async fn audio_play(
     // Bump generation first so the old progress task stops before we peel
     // chained_info (avoids a race where it sees current_done + empty chain).
     let gen = state.generation.fetch_add(1, Ordering::SeqCst) + 1;
+    // Ranged/legacy HTTP paths reset this to false in `select_play_input`.
+    state.stream_playback_armed.store(true, Ordering::SeqCst);
 
     // Manual skip onto the gapless-pre-chained track: reuse raw bytes (no HTTP;
     // preload cache was already consumed when the chain was built). Otherwise
@@ -325,7 +327,8 @@ pub async fn audio_play(
     // without an underrun on the very first period.
     // Standard mode: no pre-fill needed — default 44.1/48 kHz quantum is small.
     let needs_prefill = hi_res_enabled && output_rate > 48_000;
-    if needs_prefill {
+    let defer_playback_start = !state.stream_playback_armed.load(Ordering::Relaxed);
+    if needs_prefill || defer_playback_start {
         sink.pause();
     }
 
@@ -372,7 +375,9 @@ pub async fn audio_play(
         if state.generation.load(Ordering::SeqCst) != gen {
             return Ok(()); // skipped during pre-fill — abort silently
         }
-        sink.play();
+        if !defer_playback_start {
+            sink.play();
+        }
     }
 
     swap_in_new_sink(&state, SinkSwapInputs {
@@ -386,7 +391,24 @@ pub async fn audio_play(
         actual_fade_secs,
     });
 
-    app.emit("audio:playing", duration_secs).ok();
+    if defer_playback_start {
+        {
+            let mut cur = state.current.lock().unwrap();
+            cur.play_started = None;
+            cur.paused_at = Some(0.0);
+        }
+        spawn_legacy_stream_start_when_armed(
+            gen,
+            state.generation.clone(),
+            state.stream_playback_armed.clone(),
+            state.samples_played.clone(),
+            state.current.clone(),
+            app.clone(),
+            duration_secs,
+        );
+    } else {
+        app.emit("audio:playing", duration_secs).ok();
+    }
 
     // ── Progress + ended detection ────────────────────────────────────────────
     spawn_progress_task(
@@ -403,6 +425,7 @@ pub async fn audio_play(
         state.current_channels.clone(),
         state.gapless_switch_at.clone(),
         state.current_playback_url.clone(),
+        state.stream_playback_armed.clone(),
     );
 
     Ok(())