feat: add pycpp pybind11 wrapper layer for geometry types

Add pycpp/ directory providing thin pybind11 wrappers for shapelycpp
native C++ geometry types, following the numpypy/pycpp pattern.

pycpp/geometry_py.h:
  - Factory functions: point(f64/f32), linestring(f64/f32), polygon(f64/f32),
    linearring — accept Python list of coords
  - Cross-type distance: distance_pt_ls, distance_pt_poly, distance_ls_poly,
    distance_poly_ls
  - Cross-type intersects: intersects_ls_poly, intersects_poly_poly
  - All 9 predicate pair combinations (pt↔pt, pt↔ls, pt↔poly, ls↔pt, etc.)
  - Centroid, project, interpolate, intersection_area, polygon_exterior
  - BIND_PREDS / BIND_ACCESSORS macros for consistent class binding

pycpp/ops_py.h:
  - nearest_points_poly_ls, nearest_points_ls_pt

tests/module.cpp:
  - Refactored from 400→223 lines: uses pycpp wrappers instead of
    hand-written free functions and macros
  - Serves as reference example for consuming projects

README: updated project structure to document pycpp/ directory

Author: peng.li24

README.md

@@ -135,11 +135,14 @@ shapelycpp/
 │   │   └── polygon.h
 │   └── ops/
 │       └── nearest_points.h
+├── pycpp/              # pybind11 wrappers (thin layer: Python types → native C++)
+│   ├── geometry_py.h   #   factories, cross-type predicates, distance, accessors
+│   └── ops_py.h        #   nearest_points
 ├── example/            # native C++ usage examples
 │   ├── CMakeLists.txt
 │   └── main.cpp
 ├── tests/              # Python precision alignment tests
-│   ├── module.cpp      # pybind11 test module
+│   ├── module.cpp      # pybind11 test module (uses pycpp wrappers)
 │   ├── conftest.py     # pytest fixtures (float64/float32 make, CppFactory)
 │   ├── utils.py        # coordinate generators & comparison helpers
 │   └── test_api.py     # unified precision alignment tests (float64/float32)