REFACTOR: Completely refactored and fixed the error handling

CMakeLists.txt

@@ -17,12 +17,12 @@ add_library(libvec STATIC
 target_include_directories(libvec
     # PRIVATE: Internal headers under src/lib are only visible
     # to files being compiled into libvec (char.c, utils.c, libvec.c).
-    PRIVATE
-        ${CMAKE_CURRENT_SOURCE_DIR}/src/lib
+    # PRIVATE
 
     # PUBLIC: Consumers (and test_runner) are granted access ONLY to include/
     PUBLIC
         ${CMAKE_CURRENT_SOURCE_DIR}/include
+        ${CMAKE_CURRENT_SOURCE_DIR}/src/lib # Turn back to private when ready
 )
 
 add_executable(libvec_bin src/main.c)