refactor: merge pycpp/_py.h into numpy/, delete pycpp/ umbrella

- Move init_py.h, elementwise_py.h, reduce_py.h, manipulation_py.h,
  io_py.h, linalg_py.h from pycpp/ → numpy/
- Delete pycpp/pycpp.h (umbrella with non-numpy name)
- Delete empty pycpp/ directory
- CMakeLists.txt: remove pycpp install block, _py.h files auto-installed
  with numpy/ directory wildcard
- tests/module.cpp: replace #include "pycpp.h" with individual _py.h includes

Now all headers live under a single numpy/ tree:
  #include <numpycpp/numpy/numpy.h>        # C++ core
  #include <numpycpp/numpy/init_py.h>      # pybind11 wrappers

Author: peng.li24

CMakeLists.txt

@@ -44,10 +44,6 @@ endif()
 install(DIRECTORY numpy
     DESTINATION include/numpycpp
     FILES_MATCHING PATTERN "*.h"
-)
-install(DIRECTORY pycpp
-    DESTINATION include/numpycpp
-    FILES_MATCHING PATTERN "*.h"
     PATTERN ".pytest_cache" EXCLUDE
 )
 

pycpp/elementwise_py.h numpy/elementwise_py.hpycpp/elementwise_py.h renamed to numpy/elementwise_py.h

@@ -4,7 +4,12 @@
 
 #include <pybind11/pybind11.h>
 #include <pybind11/stl.h>
-#include "pycpp.h"
+#include "numpy/init_py.h"
+#include "numpy/elementwise_py.h"
+#include "numpy/reduce_py.h"
+#include "numpy/manipulation_py.h"
+#include "numpy/io_py.h"
+#include "numpy/linalg_py.h"
 
 namespace py = pybind11;