Fix Windows DLL loading for Python 3.8+

[Copilot]

What problem does this PR address?

Python 3.8+ restricts DLL loading on Windows to specific directories for security. Extension module srreal_ext cannot find dependencies (libdiffpy.dll, libObjCryst.dll, Boost, GSL) installed in %CONDA_PREFIX%\Library\bin, causing import failures.

What should the reviewer(s) do?

Review the Windows-specific initialization added to __init__.py and verify:

• Logic only executes on Windows with Python 3.8+
• No impact on Linux/macOS platforms
• Test coverage is adequate

Changes:

• src/diffpy/srreal/__init__.py: Add os.add_dll_directory() call for %CONDA_PREFIX%\Library\bin before extension imports
• tests/test_dll_loading.py: Verify import succeeds and Windows DLL handling works across platforms

Example:

# Before: ImportError on Windows
from diffpy.srreal import pdfcalculator

# After: Imports successfully
from diffpy.srreal import pdfcalculator
pc = PDFCalculator()

Original prompt

---

This section details on the original issue you should resolve

<issue_title>srreal DLL not loading properly on windows</issue_title>
<issue_description>### Problem
I am getting this when I try and run tests in pdfbl-sequential on windows:

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    #!/usr/bin/env python
    ##############################################################################
    #
    # diffpy.srreal     by DANSE Diffraction group
    #                   Simon J. L. Billinge
    #                   (c) 2010 The Trustees of Columbia University
    #                   in the City of New York.  All rights reserved.
    #
    # File coded by:    Pavol Juhas
    #
    # See AUTHORS.txt for a list of people who contributed.
    # See LICENSE_DANSE.txt for license information.
    #
    ##############################################################################
    """
    Top-level classes for PDF calculation:
        DebyePDFCalculator -- simulate PDF by evaluating Debye sum in Q-space
        PDFCalculator      -- calculate PDF by peak summation in real space
    """

>   from diffpy.srreal.srreal_ext import (
        DebyePDFCalculator,
        PDFCalculator,
        fftftog,
        fftgtof,
    )
E   ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.

..\..\micromamba\envs\pdfbl\Lib\site-packages\diffpy\srreal\pdfcalculator.py:21: ImportError
====================================================== short test summary info ======================================================
FAILED tests/test_pdfadapter.py::test_pdfadapter - ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
FAILED tests/test_sequential_cmi_runner.py::test_load_inputs_bad - ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
FAILED tests/test_sequential_cmi_runner.py::test_run_sequential_cmi_runner - ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
FAILED tests/test_sequential_cmi_runner.py::test_data_for_plot - ImportError: DLL load failed while importing srreal_ext: The specified module could not be found.
========================================================= 4 failed in 7.15s =========================================================

Proposed solution

Figure out why and fix it. Here is the env list:

List of packages in environment: "C:\\Users\\simon\\micromamba\\envs\\pdfbl"

  Name                Version       Build                  Channel
------------------------------------------------------------------------
  arrow               1.4.0         py312haa95532_0        pkgs/main
  bg-mpl-stylesheets  0.5.1         pyhff2d567_0           conda-forge
  binaryornot         0.4.4         pyhd3eb1b0_1           pkgs/main
  blas                1.0           mkl                    pkgs/main
  brotlicffi          1.2.0.0       py312h885b0b7_0        pkgs/main
  bzip2               1.0.8         h2bbff1b_6             pkgs/main
  ca-certificates     2025.12.2     haa95532_0             pkgs/main
  cairo               1.18.4        he9e932c_0             pkgs/main
  certifi             2026.01.04    py312haa95532_0        pkgs/main
  cffi                2.0.0         py312h02ab6af_1        pkgs/main
  cfgv                3.5.0         py312haa95532_0        pkgs/main
  chardet             5.2.0         py312haa95532_0        pkgs/main
  charset-normalizer  3.4.4         py312haa95532_0        pkgs/main
  click               8.2.1         py312haa95532_1        pkgs/main
  codecov             2.1.11        pyhd3eb1b0_0           pkgs/main
  colorama            0.4.6         py312haa95532_0        pkgs/main
  contourpy           1.3.3         py312h214f63a_0        pkgs/main
  cookiecutter        2.6.0         py312haa95532_1        pkgs/main
  coverage            7.11.0        py312h630b2a1_1        pkgs/main
  cycler              0.12.1        py312haa95532_0        pkgs/main
  diffpy.cmi          3.1.2         pyhd8ed1ab_0           conda-forge
  diffpy.srfit        3.2.0         pyhd8ed1ab_0           conda-forge
  diffpy.srreal       1.4.0         py312h0e1366e_3        conda-forge
  diffpy.structure    3.3.1         pyhd8ed1ab_0           conda-forge
  diffpy.utils        3.7.1         pyhd8ed1ab_0           conda-forge
  distlib             0.4.0         py312h02ab6af_0        pkgs/main
  dlfcn-win32         1.4.2         hac47afa_0             conda-forge
  expat               2.7.4         hd7fb8db_0             pkgs/main
  filelock            3.20.3        py312haa95532_0        pkgs/main
  flake8              7.3.0         py312haa95532_0        pkgs/main
  fontconfig          2.15.0        hd211d86_0             pkgs/main
  fonttools           4.61.0        py312h02ab6af_0        pkgs/main
  freetype            2....

</details>



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- Fixes diffpy/diffpy.srreal#81

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