fix(audio): read-ahead the backing track off the RT audio thread (#60)

* fix(audio): read-ahead the backing track off the RT audio thread

The backing AudioTransportSource was set up with no read-ahead buffer and
no reader thread — setSource(src, 0, nullptr, rate) — so the realtime audio
callback decoded the backing file synchronously inside getNextAudioBlock on
every block while a song plays. Any disk seek or codec spike (worst on
compressed formats) then blew the block's realtime budget, producing
underruns heard as glitches / brief mutes.

Interpose a juce::TimeSliceThread with 32768 source frames (~0.68 s @ 48 kHz)
of look-ahead so decode happens off the audio thread. The thread is declared
before backingTransport (so it is destroyed after it — the transport's
BufferingAudioSource holds a pointer to it) and started once in the ctor.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs(audio): document the bounded BufferingAudioSource lock residual

Codex review: readBufferSection() holds callbackLock across a refill
chunk decode and the RT callback takes the same lock. Accepted — the
window is bounded (2048-frame chunks) and only hit mid-refill, vs. the
old guaranteed synchronous decode every block; note it in the comment so
nobody mistakes the transport stack for fully RT-safe.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Jafz2001 <ignacio.fritis@mundotelecomunicaciones.cl>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

src/audio/AudioEngine.h

@@ -467,6 +467,11 @@ private:
     std::atomic<float> outputPeak{0.0f};
 
     // Backing track
+    // Read-ahead worker that fills the transport's buffer off the audio thread
+    // (see loadBackingTrack). Declared BEFORE backingTransport so it is destroyed
+    // AFTER it — the transport's BufferingAudioSource holds a pointer to this
+    // thread and must be torn down before the thread goes away.
+    juce::TimeSliceThread backingReadThread { "BackingReadAhead" };
     std::unique_ptr<juce::AudioFormatReaderSource> backingSource;
     std::unique_ptr<juce::AudioTransportSource> backingTransport;
     signalsmith::stretch::SignalsmithStretch<float> backingStretch;