refactor: extract psysonic-audio crate (M3/7)

Moves all audio playback code (Symphonia decode, rodio output, HTTP
streaming, gapless, previews, and the seven stream/ source-type
submodules from the prior split) out of the top crate into a new
psysonic-audio crate.

  crates/psysonic-audio/                  new lib crate, depends on
                                          psysonic-core + psysonic-analysis
    src/{engine,helpers,decode,…}.rs      flattened layout (no more
                                          extra audio/ namespace level)
    src/stream/                           seven submodules from M0
    src/lib.rs                            re-exports macros from
                                          psysonic-core and the public
                                          API surface

The audio↔analysis edges identified in the dep survey are now real
crate deps (audio depends on analysis directly: AnalysisCache reads,
recommended_gain_for_target, submit_analysis_cpu_seed). Only the
analysis→audio back-edge goes through the PlaybackQueryHandle port
registered in M2.

Cross-crate ref migrations applied via batch sed:
  crate::audio::*               → crate::*       (intra-crate)
  crate::analysis_cache::*      → psysonic_analysis::analysis_cache::*
  crate::submit_analysis_cpu_seed → psysonic_analysis::analysis_runtime::*
  crate::subsonic_wire_user_agent → psysonic_core::user_agent::*

Top crate keeps `crate::audio::*` paths working via
`pub use psysonic_audio as audio;` — lib_commands/cli callers untouched.
`stop_audio_engine` (mac process-exit cleanup) moved into the audio
crate as `pub fn stop_audio_engine` since it reaches AudioEngine
internals; tray.rs now re-exports the moved fn.

Two small visibility promotions in engine.rs:
  pub(crate) fn analysis_track_id_is_current_playback   → pub
  pub(crate) fn ranged_loudness_backfill_should_defer    → pub

Behaviour preserving. Cargo check + clippy --workspace clean.

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

@@ -0,0 +1,219 @@
+//! Per-generation progress + ended-detection task. Spawned once per
+//! `audio_play` / `audio_play_radio` invocation, the task ticks at 100 ms,
+//! emits `audio:progress` (throttled), handles the gapless transition
+//! when the current source exhausts and a chained successor is queued,
+//! and finally emits `audio:ended` when no successor exists.
+
+use std::sync::atomic::{AtomicBool, AtomicU32, AtomicU64, Ordering};
+use std::sync::{Arc, Mutex};
+use std::time::{Duration, Instant};
+
+use tauri::{AppHandle, Emitter};
+
+use super::engine::AudioCurrent;
+use super::helpers::{ramp_sink_volume, ProgressPayload, MASTER_HEADROOM};
+use super::state::ChainedInfo;
+
+/// Spawns the per-generation progress + ended-detection task.
+///
+/// The task owns a local `done: Arc<AtomicBool>` reference that starts as
+/// the current track's done flag. When the progress task detects that the
+/// done flag is set AND `chained_info` has data, it swaps `done` to the
+/// chained source's flag and transitions state — all without creating a new
+/// task or changing the generation counter.
+///
+/// Key changes from the previous implementation:
+///   • 100 ms tick (was 500 ms) — halves worst-case event latency
+///   • Position from atomic sample counter (no wall-clock drift)
+///   • Immediate `audio:track_switched` event at decoder boundary
+///   • `audio:ended` only fires when no chained successor exists
+pub(super) fn spawn_progress_task(
+    gen: u64,
+    gen_counter: Arc<AtomicU64>,
+    current_arc: Arc<Mutex<AudioCurrent>>,
+    chained_arc: Arc<Mutex<Option<ChainedInfo>>>,
+    crossfade_enabled_arc: Arc<AtomicBool>,
+    crossfade_secs_arc: Arc<AtomicU32>,
+    initial_done: Arc<AtomicBool>,
+    app: AppHandle,
+    samples_played: Arc<AtomicU64>,
+    sample_rate_arc: Arc<AtomicU32>,
+    channels_arc: Arc<AtomicU32>,
+    gapless_switch_at: Arc<AtomicU64>,
+    current_playback_url: Arc<Mutex<Option<String>>>,
+) {
+    // Keep progress aligned with audible output (ALSA/PipeWire/Pulse queue) on
+    // Linux; mirrors the quantum policy used for stream open/reopen plus a small
+    // scheduler/mixer cushion so the UI doesn't run ahead. Other platforms have
+    // their own latency reporting paths and don't need the compensation here.
+    #[cfg(target_os = "linux")]
+    fn estimated_output_latency_secs(sample_rate_hz: f64) -> f64 {
+        let rate = sample_rate_hz.max(1.0);
+        let frames = if rate > 48_000.0 { 8192.0 } else { 4096.0 };
+        (frames / rate) + 0.012
+    }
+    #[cfg(not(target_os = "linux"))]
+    fn estimated_output_latency_secs(_sample_rate_hz: f64) -> f64 {
+        0.0
+    }
+
+    // Keep near-end detection at 100 ms, but throttle progress IPC to webview.
+    const PROGRESS_EMIT_MIN_MS: u64 = 1500;
+    const PROGRESS_EMIT_MIN_DELTA_SECS: f64 = 0.9;
+
+    tokio::spawn(async move {
+        let mut near_end_ticks: u32 = 0;
+        // Local done-flag reference; swapped on gapless transition.
+        let mut current_done = initial_done;
+        // Local sample counter; swapped to chained source's counter on transition.
+        let mut samples_played = samples_played;
+        let mut last_progress_emit_at = Instant::now() - Duration::from_millis(PROGRESS_EMIT_MIN_MS);
+        let mut last_progress_emit_pos = -1.0f64;
+        let mut last_progress_emit_paused = false;
+
+        loop {
+            // 100 ms tick keeps near-end detection timely for crossfade/gapless
+            // handoff while frontend still interpolates smoothly via rAF.
+            tokio::time::sleep(Duration::from_millis(100)).await;
+
+            if gen_counter.load(Ordering::SeqCst) != gen {
+                break;
+            }
+
+            // ── Gapless transition detection ─────────────────────────────────
+            // If the current source is exhausted AND we have a chained track
+            // ready, transition seamlessly: swap tracking state, emit
+            // audio:track_switched for the new track, and continue the loop.
+            if current_done.load(Ordering::SeqCst) {
+                // Radio (dur == 0): stream exhausted / connection dropped → stop.
+                let cur_dur = current_arc.lock().unwrap().duration_secs;
+                if cur_dur <= 0.0 {
+                    crate::app_eprintln!("[radio] current_done fired → emitting audio:ended (dur=0)");
+                    gen_counter.fetch_add(1, Ordering::SeqCst);
+                    app.emit("audio:ended", ()).ok();
+                    break;
+                }
+
+                let chained = chained_arc.lock().unwrap().take();
+                if let Some(info) = chained {
+                    // Swap to the chained source's done flag.
+                    current_done = info.source_done;
+
+                    // Swap to the chained source's sample counter.
+                    // The chained CountingSource increments its own Arc,
+                    // so we must rebind our local reference to it —
+                    // a one-time value copy would go stale immediately.
+                    samples_played = info.sample_counter;
+
+                    // Update tracking state and apply the chained track's
+                    // effective volume. Deferred from `audio_chain_preload`
+                    // (which runs ~30 s before the current track ends) to
+                    // avoid changing loudness of the still-playing current
+                    // track. `Sink::set_volume` affects the whole Sink, so it
+                    // must only be called at the boundary, not at preload.
+                    {
+                        let mut cur = current_arc.lock().unwrap();
+                        let prev_effective = (cur.base_volume * cur.replay_gain_linear * MASTER_HEADROOM).clamp(0.0, 1.0);
+                        cur.replay_gain_linear = info.replay_gain_linear;
+                        cur.base_volume = info.base_volume;
+                        cur.duration_secs = info.duration_secs;
+                        cur.seek_offset = 0.0;
+                        cur.play_started = Some(Instant::now());
+                        if let Some(sink) = &cur.sink {
+                            let effective = (cur.base_volume * cur.replay_gain_linear * MASTER_HEADROOM).clamp(0.0, 1.0);
+                            ramp_sink_volume(Arc::clone(sink), prev_effective, effective);
+                        }
+                    }
+
+                    *current_playback_url.lock().unwrap() = Some(info.url.clone());
+
+                    // Record the gapless switch timestamp for ghost-command guard.
+                    let switch_ts = std::time::SystemTime::now()
+                        .duration_since(std::time::UNIX_EPOCH)
+                        .unwrap_or_default()
+                        .as_millis() as u64;
+                    gapless_switch_at.store(switch_ts, Ordering::SeqCst);
+
+                    // Emit the new track_switched event — this is immediate,
+                    // not delayed by 500 ms like the old audio:playing was.
+                    app.emit("audio:track_switched", info.duration_secs).ok();
+                    near_end_ticks = 0;
+                    continue;
+                }
+                // Current source exhausted but no chain queued — the Sink is
+                // likely draining; audio:ended will fire on the next tick via
+                // the near-end logic below.
+            }
+
+            // ── Position from atomic sample counter ──────────────────────────
+            let rate = sample_rate_arc.load(Ordering::Relaxed) as f64;
+            let ch = channels_arc.load(Ordering::Relaxed) as f64;
+            let samples = samples_played.load(Ordering::Relaxed) as f64;
+            let divisor = (rate * ch).max(1.0);
+
+            // Read playback snapshot under a single lock to minimize contention
+            // with seek/play/pause commands that also touch `current`.
+            let (dur, paused_at) = {
+                let cur = current_arc.lock().unwrap();
+                (cur.duration_secs, cur.paused_at)
+            };
+            let is_paused = paused_at.is_some();
+
+            let pos_raw = if let Some(p) = paused_at {
+                p
+            } else {
+                (samples / divisor).min(dur.max(0.001))
+            };
+            let progress_latency = if is_paused {
+                0.0
+            } else {
+                estimated_output_latency_secs(rate)
+            };
+            let pos = (pos_raw - progress_latency).max(0.0);
+
+            let now = Instant::now();
+            let should_emit_progress = if is_paused != last_progress_emit_paused {
+                true
+            } else if now.duration_since(last_progress_emit_at) >= Duration::from_millis(PROGRESS_EMIT_MIN_MS) {
+                true
+            } else {
+                (pos - last_progress_emit_pos).abs() >= PROGRESS_EMIT_MIN_DELTA_SECS
+            };
+            if should_emit_progress {
+                app.emit("audio:progress", ProgressPayload { current_time: pos, duration: dur }).ok();
+                last_progress_emit_at = now;
+                last_progress_emit_pos = pos;
+                last_progress_emit_paused = is_paused;
+            }
+
+            if is_paused {
+                continue;
+            }
+
+            let cf_enabled = crossfade_enabled_arc.load(Ordering::Relaxed);
+            let cf_secs = f32::from_bits(crossfade_secs_arc.load(Ordering::Relaxed)).clamp(0.5, 12.0) as f64;
+            let end_threshold = if cf_enabled { cf_secs.max(1.0) } else { 1.0 };
+
+            if dur > end_threshold && pos_raw >= dur - end_threshold {
+                near_end_ticks += 1;
+                // At 100 ms ticks, 10 ticks ≈ 1 s — equivalent to the old 2×500ms.
+                if near_end_ticks >= 10 {
+                    // If a gapless chain is pending, the source hasn't
+                    // exhausted yet — duration_hint (integer seconds from
+                    // Subsonic) is shorter than the actual audio content.
+                    // Don't emit audio:ended; let the gapless transition
+                    // handle it when current_done fires.
+                    let has_chain = chained_arc.lock().unwrap().is_some();
+                    if has_chain {
+                        continue;
+                    }
+                    gen_counter.fetch_add(1, Ordering::SeqCst);
+                    app.emit("audio:ended", ()).ok();
+                    break;
+                }
+            } else {
+                near_end_ticks = 0;
+            }
+        }
+    });
+}