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.

include/Database.h

@@ -1,6 +1,6 @@
 #pragma once
 
-#include "Metadata.h"
+#include "Containers.h"
 
 #include <sqlite3.h>
 
@@ -10,8 +10,9 @@ class Database {
     public:
         Database( const char* filename, sqlite3** db );
         ~Database();
-        void fetch( const std::string& query );
-        void fetchAlbum( const std::string& album );
+        void fetch( const std::string& query ) const;
+        void fetchAlbum( const std::string& album ) const;
+        void fetchAll() const;
         void addSong( );
     private:
         sqlite3* db_;