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/preload_commands.rs

@@ -0,0 +1,67 @@
+//! Background audio_preload: fetch the next track's bytes ahead of time
+//! and seed the analysis cache. Distinct from `audio_chain_preload`
+//! (which constructs the gapless source chain) and `audio_play` (which
+//! starts playback). All three live in this audio submodule.
+
+use std::sync::atomic::Ordering;
+use std::time::Duration;
+
+use tauri::{AppHandle, Emitter, State};
+
+use super::engine::{audio_http_client, AudioEngine};
+use super::helpers::{analysis_cache_track_id, same_playback_target};
+use super::state::PreloadedTrack;
+
+#[tauri::command]
+pub async fn audio_preload(
+    url: String,
+    duration_hint: f64,
+    analysis_track_id: Option<String>,
+    app: AppHandle,
+    state: State<'_, AudioEngine>,
+) -> Result<(), String> {
+    {
+        let preloaded = state.preloaded.lock().unwrap();
+        if preloaded.as_ref().is_some_and(|p| same_playback_target(&p.url, &url)) {
+            let _ = app.emit("audio:preload-ready", url.clone());
+            return Ok(());
+        }
+    }
+    // Throttle: wait 8 s before starting the background download so it does not
+    // compete with the decode + sink-feed work of the just-started current track.
+    // If the user skips during the wait the generation counter changes and we abort.
+    let gen_snapshot = state.generation.load(Ordering::Relaxed);
+    tokio::time::sleep(Duration::from_secs(8)).await;
+    if state.generation.load(Ordering::Relaxed) != gen_snapshot {
+        return Ok(());
+    }
+    let data: Vec<u8> = if let Some(path) = url.strip_prefix("psysonic-local://") {
+        tokio::fs::read(path).await.map_err(|e| e.to_string())?
+    } else {
+        let response = audio_http_client(&state).get(&url).send().await.map_err(|e| e.to_string())?;
+        if !response.status().is_success() {
+            return Ok(());
+        }
+        response.bytes().await.map_err(|e| e.to_string())?.into()
+    };
+    let _ = duration_hint; // kept in API for compatibility
+    let logical_trim = analysis_track_id
+        .as_ref()
+        .map(|s| s.trim().to_string())
+        .filter(|s| !s.is_empty());
+    if let Some(track_id) = analysis_cache_track_id(logical_trim.as_deref(), &url) {
+        crate::app_deprintln!(
+            "[stream] audio_preload: bytes ready track_id={} size_mib={:.2} — invoking full-track analysis",
+            track_id,
+            data.len() as f64 / (1024.0 * 1024.0)
+        );
+        let high = crate::engine::analysis_track_id_is_current_playback(&state, &track_id);
+        if let Err(e) = psysonic_analysis::analysis_runtime::submit_analysis_cpu_seed(app.clone(), track_id.clone(), data.clone(), high).await {
+            crate::app_eprintln!("[analysis] preload seed failed for {}: {}", track_id, e);
+        }
+    }
+    let url_for_emit = url.clone();
+    *state.preloaded.lock().unwrap() = Some(PreloadedTrack { url, data });
+    let _ = app.emit("audio:preload-ready", url_for_emit);
+    Ok(())
+}