refactor(audio): lift PlayInput → BuiltSource dispatch into helper

Pull the 60-LOC match in audio_play that turned a PlayInput into a
fully-wrapped rodio source out of audio_play and into
play_input::build_source_from_play_input. Returns a small PlaybackSource
struct holding both the BuiltSource and a `is_seekable` flag (only
the Streaming variant is non-seekable).

Behaviour preserved verbatim — same build_source / build_streaming_source
calls, same target_rate=0 (no app-level resampling), same
spawn_blocking decoder build for Seekable+Streaming, same error path
(`audio:error` emit + propagate).

audio/commands.rs: 642 → 600 LOC. The remaining audio_play body now
reads top-to-bottom as: ghost guard → preview clear → gapless pre-chain
check → bump generation → reuse-bytes prep → URL pin → format hint →
**select_play_input** → gen check → loudness/RG via gain_inputs →
crossfade prep → **build_source_from_play_input** → stream rate
switching → sink construction + prefill → swap sink → progress task.

src-tauri/src/audio/commands.rs

@@ -10,11 +10,13 @@ use rodio::Player;
 use rodio::Source;
 use tauri::{AppHandle, Emitter, State};
 
-use super::decode::{build_source, build_streaming_source, SizedDecoder};
+use super::decode::build_source;
 use super::engine::{audio_http_client, AudioEngine};
 use super::helpers::*;
 use super::ipc::{maybe_emit_normalization_state, NormalizationStatePayload};
-use super::play_input::{select_play_input, url_format_hint, PlayInput, PlayInputContext};
+use super::play_input::{
+    build_source_from_play_input, select_play_input, url_format_hint, PlayInputContext,
+};
 use super::preview::preview_clear_for_new_main_playback;
 use super::progress_task::spawn_progress_task;
 use super::state::{ChainedInfo, PreloadedTrack};
@@ -216,65 +218,22 @@ pub async fn audio_play(
     let done_flag = Arc::new(AtomicBool::new(false));
     // Reset sample counter for the new track.
     state.samples_played.store(0, Ordering::Relaxed);
-    // Always 0 — no application-level resampling. Rodio handles conversion to
-    // the output device rate internally; we let every track play at its native rate.
-    let target_rate: u32 = 0;
-    let mut new_is_seekable = true;
-    let built = match play_input {
-        PlayInput::Bytes(data) => build_source(
-            data,
-            duration_hint,
-            state.eq_gains.clone(),
-            state.eq_enabled.clone(),
-            state.eq_pre_gain.clone(),
-            done_flag.clone(),
-            fade_in_dur,
-            state.samples_played.clone(),
-            target_rate,
-            format_hint.as_deref(),
-            hi_res_enabled,
-        ),
-        PlayInput::SeekableMedia { reader, format_hint, tag } => {
-            let decoder = tokio::task::spawn_blocking(move || {
-                SizedDecoder::new_streaming(reader, format_hint.as_deref(), tag)
-            })
-            .await
-            .map_err(|e| e.to_string())??;
-
-            build_streaming_source(
-                decoder,
-                duration_hint,
-                state.eq_gains.clone(),
-                state.eq_enabled.clone(),
-                state.eq_pre_gain.clone(),
-                done_flag.clone(),
-                fade_in_dur,
-                state.samples_played.clone(),
-                target_rate,
-            )
-        }
-        PlayInput::Streaming { reader, format_hint } => {
-            new_is_seekable = false;
-            let decoder = tokio::task::spawn_blocking(move || {
-                SizedDecoder::new_streaming(Box::new(reader), format_hint.as_deref(), "track-stream")
-            })
-            .await
-            .map_err(|e| e.to_string())??;
-
-            build_streaming_source(
-                decoder,
-                duration_hint,
-                state.eq_gains.clone(),
-                state.eq_enabled.clone(),
-                state.eq_pre_gain.clone(),
-                done_flag.clone(),
-                fade_in_dur,
-                state.samples_played.clone(),
-                target_rate,
-            )
-        }
-    }.map_err(|e| { app.emit("audio:error", &e).ok(); e })?;
-    state.current_is_seekable.store(new_is_seekable, Ordering::SeqCst);
+    let playback_source = build_source_from_play_input(
+        play_input,
+        &state,
+        format_hint.as_deref(),
+        done_flag.clone(),
+        fade_in_dur,
+        hi_res_enabled,
+        duration_hint,
+    )
+    .await
+    .map_err(|e| {
+        app.emit("audio:error", &e).ok();
+        e
+    })?;
+    state.current_is_seekable.store(playback_source.is_seekable, Ordering::SeqCst);
+    let built = playback_source.built;
     let source = built.source;
     let duration_secs = built.duration_secs;
     let output_rate = built.output_rate;

src-tauri/src/audio/play_input.rs

@@ -11,6 +11,7 @@ use ringbuf::{HeapCons, HeapRb};
 use symphonia::core::io::MediaSource;
 use tauri::{AppHandle, Emitter, Manager, State};
 
+use super::decode::{build_source, build_streaming_source, BuiltSource, SizedDecoder};
 use super::engine::{audio_http_client, AudioEngine};
 use super::helpers::{
     content_type_to_hint, fetch_data, format_hint_from_content_disposition,
@@ -383,3 +384,82 @@ pub(super) fn url_format_hint(url: &str) -> Option<String> {
         })
         .map(|s| s.to_lowercase())
 }
+
+/// Output of `build_source_from_play_input`: the wrapped rodio source plus
+/// whether the chosen source path is seekable (only the Streaming variant
+/// is not).
+pub(super) struct PlaybackSource {
+    pub(super) built: BuiltSource,
+    pub(super) is_seekable: bool,
+}
+
+/// Dispatch [`PlayInput`] → fully wrapped rodio source. For Bytes the full
+/// in-memory pipeline (incl. iTunSMPB scan); for SeekableMedia / Streaming
+/// the streaming variant runs the decoder build on a blocking thread.
+pub(super) async fn build_source_from_play_input(
+    play_input: PlayInput,
+    state: &State<'_, AudioEngine>,
+    format_hint: Option<&str>,
+    done_flag: Arc<AtomicBool>,
+    fade_in_dur: Duration,
+    hi_res_enabled: bool,
+    duration_hint: f64,
+) -> Result<PlaybackSource, String> {
+    // Always 0 — no application-level resampling. Rodio handles conversion to
+    // the output device rate internally; we let every track play at its native rate.
+    let target_rate: u32 = 0;
+    let mut is_seekable = true;
+    let built = match play_input {
+        PlayInput::Bytes(data) => build_source(
+            data,
+            duration_hint,
+            state.eq_gains.clone(),
+            state.eq_enabled.clone(),
+            state.eq_pre_gain.clone(),
+            done_flag,
+            fade_in_dur,
+            state.samples_played.clone(),
+            target_rate,
+            format_hint,
+            hi_res_enabled,
+        ),
+        PlayInput::SeekableMedia { reader, format_hint: media_hint, tag } => {
+            let decoder = tokio::task::spawn_blocking(move || {
+                SizedDecoder::new_streaming(reader, media_hint.as_deref(), tag)
+            })
+            .await
+            .map_err(|e| e.to_string())??;
+            build_streaming_source(
+                decoder,
+                duration_hint,
+                state.eq_gains.clone(),
+                state.eq_enabled.clone(),
+                state.eq_pre_gain.clone(),
+                done_flag,
+                fade_in_dur,
+                state.samples_played.clone(),
+                target_rate,
+            )
+        }
+        PlayInput::Streaming { reader, format_hint: stream_hint } => {
+            is_seekable = false;
+            let decoder = tokio::task::spawn_blocking(move || {
+                SizedDecoder::new_streaming(Box::new(reader), stream_hint.as_deref(), "track-stream")
+            })
+            .await
+            .map_err(|e| e.to_string())??;
+            build_streaming_source(
+                decoder,
+                duration_hint,
+                state.eq_gains.clone(),
+                state.eq_enabled.clone(),
+                state.eq_pre_gain.clone(),
+                done_flag,
+                fade_in_dur,
+                state.samples_played.clone(),
+                target_rate,
+            )
+        }
+    }?;
+    Ok(PlaybackSource { built, is_seekable })
+}