Refactor metadata and internal data structures

Replace Metadata.h with Containers.h, introducing a HashTable
implementation with SIMD acceleration and specialized track tracking.
Cleanup PlayerEngine logic by merging it into the playback flow and
refactoring several internal API signatures.

src/PlaybackEngine.cpp

@@ -69,7 +69,7 @@ void PE::seek ( ma_uint64 frame ) {
     ma_decoder_seek_to_pcm_frame ( &state.decoder, frame );
 }
 
-auto PE::getPosition () {
+uint64_t PE::getPosition () {
     ma_decoder_get_cursor_in_pcm_frames ( &state.decoder, &cursor_ );
     return cursor_;
 }