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.rs

@@ -1,7 +1,6 @@
 // Prevents additional console window on Windows in release, DO NOT REMOVE!!
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
 
-mod audio;
 pub mod cli;
 mod discord;
 mod lib_commands;
@@ -12,9 +11,7 @@ pub use psysonic_core::user_agent::{
     default_subsonic_wire_user_agent, runtime_subsonic_wire_user_agent, subsonic_wire_user_agent,
 };
 pub use psysonic_analysis::{analysis_cache, analysis_runtime};
-// `crate::submit_analysis_cpu_seed` shorthand kept so the audio modules don't
-// have to spell out `psysonic_analysis::analysis_runtime::...` until M3.
-pub(crate) use psysonic_analysis::analysis_runtime::submit_analysis_cpu_seed;
+pub use psysonic_audio as audio;
 #[cfg(target_os = "windows")]
 mod taskbar_win;
 mod tray_runtime;