Fix missing #include <cstddef> in clustering_result.hpp

GCC on Linux doesn't transitively include size_t from <vector>.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ElektrikAkar

dtwc/core/clustering_result.hpp

@@ -11,6 +11,7 @@
 
 #pragma once
 
+#include <cstddef>
 #include <vector>
 
 namespace dtwc::core {