Replace zalloc with calloc/free, add verify

Replace many uses of the project's custom zalloc/zrealloc/zfree with standard calloc/realloc/free across core containers (vector, deque, list, tree, hashtable, bitset, msort, arena, etc.). Add memory-tracking wrappers and macros under OPENCSTL_TRACER, introduce verify.h (verify macro and failure handler), begin string.h (string utilities skeleton), and adjust tracer/logging messages and diagnostic pragmas. Also include small helper and README updates (add string.h to README, adjust write_title spacing) and an updated asset image. These changes unify allocation calls, add verification utilities, and prepare for tracer-based memory instrumentation.

Author: springkim

README.md

@@ -193,7 +193,8 @@ OpenCSTL과 대응하는 C++ STL 코드입니다.
           H(hashtable.h)
       end
 
-			B(bitset.h) --> OCSTL(opencstl.h)
+	  B(bitset.h) --> OCSTL(opencstl.h)
+	  STR(string.h) --> OCSTL(opencstl.h)
       V(vector.h) --> OCSTL(opencstl.h)
       LST(list.h) --> OCSTL(opencstl.h)
       TREE(tree.h) --> OCSTL(opencstl.h)