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/codec.rs

@@ -0,0 +1,21 @@
+//! Symphonia codec registry (incl. Opus) and radio decoder factory.
+use std::sync::OnceLock;
+
+use symphonia::core::codecs::{CodecRegistry, DecoderOptions};
+
+pub(crate) fn psysonic_codec_registry() -> &'static CodecRegistry {
+    static REGISTRY: OnceLock<CodecRegistry> = OnceLock::new();
+    REGISTRY.get_or_init(|| {
+        let mut registry = CodecRegistry::new();
+        symphonia::default::register_enabled_codecs(&mut registry);
+        registry.register_all::<symphonia_adapter_libopus::OpusDecoder>();
+        registry
+    })
+}
+
+pub(crate) fn try_make_radio_decoder(
+    params: &symphonia::core::codecs::CodecParameters,
+    opts: &DecoderOptions,
+) -> Result<Box<dyn symphonia::core::codecs::Decoder>, symphonia::core::errors::Error> {
+    psysonic_codec_registry().make(params, opts)
+}