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/src/lib_commands/sync/tray.rs

@@ -1,12 +1,9 @@
-use std::sync::atomic::Ordering;
-
 use tauri::menu::{MenuBuilder, MenuItemBuilder, PredefinedMenuItem};
 use tauri::tray::{MouseButton, TrayIcon, TrayIconBuilder, TrayIconEvent};
 use tauri::{Emitter, Manager};
 #[cfg(not(target_os = "windows"))]
 use tauri::tray::MouseButtonState;
 
-use crate::audio;
 use crate::tray_runtime::{
     tray_state_icon, TrayMenuItems, TrayMenuItemsState, TrayMenuLabels, TrayMenuLabelsState,
     TrayPlaybackState, TrayState, TrayTooltip,
@@ -355,16 +352,7 @@ pub(crate) fn toggle_tray_icon(
     Ok(())
 }
 
-/// Stops the Rust audio engine cleanly (mirrors the logic in `audio_stop`).
-/// Called before process exit on macOS to ensure audio stops immediately.
-pub(crate) fn stop_audio_engine(app: &tauri::AppHandle) {
-    let engine = app.state::<audio::AudioEngine>();
-    engine.generation.fetch_add(1, Ordering::SeqCst);
-    *engine.chained_info.lock().unwrap() = None;
-    drop(engine.radio_state.lock().unwrap().take());
-    let mut cur = engine.current.lock().unwrap();
-    if let Some(sink) = cur.sink.take() { sink.stop(); }
-}
+pub(crate) use crate::audio::stop_audio_engine;
 
 /// Returns `true` if running under a tiling window manager (Hyprland, Sway, i3,
 /// bspwm, AwesomeWM, Openbox, etc.).  Detection is based on environment variables