PY_SITE_PACKAGES_PATH ignores CMAKE_INSTALL_PREFIX, so Python install fails outside a venv #6931

Wrapping/Generators/Python/CMakeLists.txt:38-79:

• sysconfig.get_path('platlib') is stored once as a CACHE STRING; a later change of CMAKE_INSTALL_PREFIX does not recompute it.
• When CMAKE_INSTALL_PREFIX != sys.prefix, an install(CODE ...) prints a warning, then the install proceeds into the interpreter's site-packages anyway.
• The failure surfaces at the first bindings file (itkConfig.py), after the C++ install has completed.

1. When the prefixes differ, default to the prefix-relative location, ${CMAKE_INSTALL_PREFIX}/<platlib relative to sys.prefix> (e.g. lib/python3.14/site-packages, Lib/site-packages on Windows), and keep the interpreter's platlib only when CMAKE_INSTALL_PREFIX equals sys.prefix (venv, conda, pixi).
2. Report the decision at configure time with message(STATUS), and replace the install-time WARNING with a configure-time one that names the variable to set.
3. Check writability of the chosen directory at configure time when it already exists, so the failure is reported before a long build.
4. Keep PY_SITE_PACKAGES_PATH as the explicit override; document it in Documentation/docs/contributing/module_workflows.md (the install-tree section mentions it as of ENH: Build, test, and wrap external modules against an installed ITK #6891) and in the Python build docs.
5. Add a CI check that installs with ITK_WRAP_PYTHON=ON into a scratch prefix whose Python is the system interpreter, so the install path is exercised outside a venv.

Item 1 changes where every non-venv Python build installs, so it needs a release note; builds inside a venv, conda, or pixi environment are unaffected.