feat: Add PlaylistSource and ResamplingNode for playlist playback

This commit implements a new audio source that reads from pmoplaylist
and streams tracks continuously, along with a resampling node to
normalize sample rates.

## New Components

### PlaylistSource (pmoaudio-ext)
- New source in pmoaudio-ext/src/sources/playlist_source.rs
- Reads from pmoplaylist ReadHandle
- Decodes tracks from audio cache (pmoaudiocache)
- Emits PCM with heterogeneous sample_rate and bit_depth
- Polls playlist when empty (configurable interval, default 100ms)
- Emits TrackBoundary markers between tracks
- Graceful shutdown with EndOfStream on stop
- Gated behind 'playlist' feature flag

**Design Philosophy:**
- Keeps each node simple (single responsibility)
- Emits raw PCM without format normalization
- Pipeline designer chooses how to handle heterogeneity
- Ideal for Radio Paradise (homogeneous streams)
- Requires ResamplingNode + ToI24Node for mixed playlists

### ResamplingNode (pmoaudio)
- Generic resampling node in pmoaudio/src/nodes/resampling_node.rs
- Normalizes variable sample rates to a target rate
- Uses libsoxr for high-quality resampling
- Automatically detects sample rate changes
- Recreates resampler as needed
- Preserves chunk type (I16/I24/I32/F32/F64)
- Quality adapts to bit depth (Medium/High/Very High)

## Architecture

PlaylistSource is placed in pmoaudio-ext to avoid circular dependencies:
- pmoaudio-ext depends on: pmoaudio, pmoplaylist, pmoaudiocache
- No reverse dependencies = clean dependency graph

## Configuration

### pmoaudio-ext/Cargo.toml
- Updated 'playlist' feature to include pmoaudiocache, pmocache, pmoflac
- Added sources module export

### pmoaudio
- Added resampling_node module
- Public export: ResamplingNode

## System Requirements

⚠️ **IMPORTANT**: libsoxr-dev must be installed for compilation

See INSTALL_NOTES.md for installation instructions per platform.

## Usage Example

```rust
// Radio Paradise (homogeneous 44.1kHz/16bit)
let mut source = PlaylistSource::new(playlist, cache);
let to_i24 = ToI24Node::new();
source.register(Box::new(to_i24));

// Mixed playlist (needs normalization)
let mut source = PlaylistSource::new(playlist, cache);
let mut resampler = ResamplingNode::new(48000);  // Force 48kHz
let to_i24 = ToI24Node::new();
source.register(Box::new(resampler));
resampler.register(Box::new(to_i24));
```

## Files Changed
- pmoaudio-ext/Cargo.toml: Update playlist feature
- pmoaudio-ext/src/lib.rs: Add sources module
- pmoaudio-ext/src/sources/mod.rs: New sources module
- pmoaudio-ext/src/sources/playlist_source.rs: New PlaylistSource (580 lines)
- pmoaudio/src/nodes/resampling_node.rs: New ResamplingNode (350 lines)
- pmoaudio/src/nodes/mod.rs: Register resampling_node
- pmoaudio/src/lib.rs: Export ResamplingNode
- INSTALL_NOTES.md: System requirements documentation

## Future Work
- GapInsertionNode (inserts silence between tracks)
- CrossfadeNode (fade-in/fade-out mixing)
- Examples (deferred until implementation validated)

pmoaudio-ext/src/lib.rs

@@ -7,7 +7,7 @@
 //! # Features
 //!
 //! - `cache-sink` : Active le `FlacCacheSink` qui encode l'audio en FLAC et le stocke dans pmoaudiocache
-//! - `playlist` : Active l'intégration avec pmoplaylist pour les sinks
+//! - `playlist` : Active l'intégration avec pmoplaylist (sources et sinks)
 //! - `all` : Active toutes les features d'un coup
 //!
 //! # Architecture
@@ -25,6 +25,12 @@
 #[cfg(feature = "cache-sink")]
 pub mod sinks;
 
+#[cfg(feature = "playlist")]
+pub mod sources;
+
 // Re-exports pour faciliter l'utilisation
 #[cfg(feature = "cache-sink")]
 pub use sinks::*;
+
+#[cfg(feature = "playlist")]
+pub use sources::*;