Fix FLAC pk collision by skipping header for pk calculation

Problem: All FLAC files with the same format (44.1kHz, stereo, 16-bit)
had identical headers and thus the same pk (071c5713d5cf485ca688832207bef0f9).
This caused the cache to think all tracks were the same file, regardless
of channel selection or actual content.

Solution: Skip the FLAC header (first 512 bytes) and calculate the pk
from bytes 512-1024 (actual audio content) instead. This ensures each
track gets a unique pk based on its actual audio data, not just its
format header.

Changes:
- Modified add_from_reader_with_pk() to read 1024 bytes instead of 512
- Use bytes 512-1024 for pk calculation when explicit_pk is None
- This works even with poor metadata (empty artist/title)
- Maintains backward compatibility with explicit_pk parameter

Fixes the issue where changing radio channel played the same song.

pmoaudio-ext/src/sinks/flac_cache_sink.rs

@@ -135,6 +135,8 @@ impl NodeLogic for FlacCacheSinkLogic {
             // Ingérer le FLAC progressivement dans le cache
             // add_from_reader lance l'ingestion en arrière-plan et retourne dès que
             // le prebuffer (512 KB) est atteint, permettant un streaming progressif
+            // Le cache skip automatiquement le header FLAC (512 octets) pour calculer le pk
+            // à partir du contenu audio, évitant les collisions entre morceaux au même format
             let collection_ref = self.collection.as_deref();
             let cache_future = self.cache.add_from_reader(
                 None,