Add TimerNode for rate limiting and improve progressive cache handling

Changes:
- Add TimerNode (pmoaudio/src/nodes/timer_node.rs): Rate-limits audio chunk flow based on timestamps with configurable max_lead_time
- Integrate TimerNode into play_and_cache.rs pipeline: PlaylistSource → TimerNode (3s pacing) → AudioSink
- Improve EOF retry in playlist_source.rs: Wait for prebuffer (512KB) before decoding, retry on temporary EOF with 200ms delay
- Export TimerNode in pmoaudio lib.rs and nodes/mod.rs

Known issue: Cache files may still be truncated when TrackBoundary arrives before pump completes flushing.
This requires allowing parallel write tasks as suggested.

pmoaudio/src/lib.rs

@@ -124,6 +124,7 @@ pub use nodes::{
     flac_file_sink::{FlacFileSink, FlacFileSinkStats},
     http_source::HttpSource,
     resampling_node::ResamplingNode,
+    timer_node::TimerNode,
     AudioError, AudioNode, TypedAudioNode,
 };