[cadenmyers13]

What problem does this PR address?

Starts the migration of libdiffpy to the scikit-package standard. libdiffpy is a pure C++ library built with SCons, so only the non-Python parts of the template apply. The SCons build is unchanged.

Changes so far:

• Generated the scikit-package template and moved the existing code into it. Source, SCons build files, tests, and examples keep their original paths.
• Filled in requirements/ with the C++ build dependencies (cxx-compiler, scons, libboost-devel, gsl, libobjcryst, cxxtest).
• Converted README.md, LICENSE.txt, AUTHORS.txt, and CHANGELOG.md to .rst, keeping the project content. Updated the license and authors references in all source file headers.
• Removed the old conda-recipe/ used for the retired diffpy channel and the template setup.py.

tests ran with scons test and all tests pass

Still to do: pre-commit, replacing the GitHub workflows with SCons-based CI, and merging doc/manual into docs/

[cadenmyers13]

• Place all source code in scikit-package directory structure
• Create CI that does everything that scikit-package CI does but for C++ code
• Remove python specific modules liek version.py and the testing dir used for pytest
• Decide if we need codecov, is there a replacement?
• Remove python specific pre-commit config tools like .flake8, black, etc

Claude generated:

Versioning and release

• Decide on the tag format (1.4.2 vs v1.4.2) and make these agree: release-scripts tag check, git describe --match in site_scons/libdiffpybuildutils.py, the tag: v regex in getversion(), and the feedstock source URL
• Remove the manual FALLBACK_VERSION bump (or add it to the release checklist), so a release doesn't fail the SCons version check
• Make sure the first release from migrated main is versioned after 1.4.1 (1.4.1 isn't in main's history)
• Replace build-wheel-release-upload.yml with a workflow calling _check-tag-privilege.yml + _release-github.yml (no wheel, no PyPI)
• Adapt .github/ISSUE_TEMPLATE/release_checklist.md, which assumes PyPI, python -m build and twine
• Update conda-forge/libdiffpy-feedstock at the next release: license_file: LICENSE.rst, source URL tag format, and maybe run scons test in the feedstock tests (it only checks that the library file exists)

CI details

• Linux, macOS, and Windows matrix (the feedstock builds Windows with dlfcn-win32)
• Pin Python ≤ 3.14 for cxxtest
• Handle the conda rpath in CI (a CI-only sconscript.local, or have SConstruct detect CONDA_PREFIX)
• Compile and run examples/testlib.cpp against the installed library (the C++ version of the scikit-package "run the examples" check)
• Fix the README |CI| badge filename once the workflow exists

Documentation

• Merge doc/manual/source and docs/development_instructions.md into docs/source, then remove doc/
• Replace docs/source/license.rst (still the template's BSD text) with the real license
• Get the docs version from site_scons/libdiffpybuildutils.py instead of importlib.metadata
• Replace the Python "Package API" page with C++ API docs (Doxygen + Breathe), or drop it
• Docs publishing workflow (_release-docs.yml runs pip install -e ., so it needs a custom version)
• Update the README pre-commit paragraph

Leftover template and legacy files

• Cut pyproject.toml down to the [tool.*] sections pre-commit uses
• Remove MANIFEST.in and requirements/pip.txt
• Remove .travis.yml and .travis-sconscript.local
• Set up codespell for C++ and data files (skip *.dat, *.cif, *.stru, and add an ignore-words list)

Process

• Add a news item to every PR (check-news-item.yml already works)
• Open a scikit-package issue for C++ library support, and propose a reusable C++ test workflow for release-scripts

Separate issues (not blocking)

• scons -j parallel build sometimes produces empty test object files
• CxxTest is unmaintained and tied to Python versions; consider moving to GoogleTest or Boost.Test