feat: add support for python geometry plugins

[wdconinc]

This PR adds a DECLARE_DETELEMENT plugin DD4hep_PythonDetector that delegates geometry construction to a Python function specified by module/function/pythonpath XML attributes. This enables geometry to be distributed as pure Python packages without any per-project C++ compilation.

This PR also adds DDCore/python/dd4hep_geo_plugin.py providing the cppyy workarounds and type aliases needed by plugin authors (to be imported with from dd4hep_geo_plugin import *).

This PR also ports the IronCylinder example (the simplest I could find) from a C++ example to a minimal Python example, and includes it as a test.

BEGINRELEASENOTES

• feat: add support for python geometry plugins

ENDRELEASENOTES

Note: This is for discussion; as I understand it from discussion with @jmcarcell there may be more computational intensive stuff done in the geometry plugins in FCC so this may impact initialization time in production and may add a translation step in project transition from prototype to production. In the case of EIC we have a challenge where incoming workforce is generally more familiar with python and this address that.

[github-actions]

Test Results

   18 files     18 suites   6h 16m 29s ⏱️
  358 tests   358 ✅ 0 💤 0 ❌
3 152 runs  3 152 ✅ 0 💤 0 ❌

Results for commit 67cf7c3.

♻️ This comment has been updated with latest results.

[wdconinc]

Aha, interesting, https://github.com/wdconinc/DD4hep/actions/runs/25173235648/job/73798348230 fails since this requires ROOT v6.34 for TPython::SwapWithObjAtAddr. I'll make this functionality conditional on ROOT version.

[Copilot]

Pull request overview

Adds a new detector factory that delegates DD4hep geometry construction to Python, plus a small helper module and an example/test demonstrating the workflow.

Changes:

• Add DD4hep_PythonDetector (DECLARE_DETELEMENT) that imports a Python module/function specified in compact XML and returns a DetElement built in Python.
• Add DDCore/python/dd4hep_geo_plugin.py helper module (cppyy workarounds + commonly used DD4hep type aliases) for plugin authors.
• Add a Python port of the IronCylinder example + compact XML and wire it into ClientTests with a ROOT/TPython availability guard.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
examples/ClientTests/python/IronCylinder_python_geo.py	New Python geometry factory function for the IronCylinder example.
examples/ClientTests/compact/IronCylinder_python.xml	New compact XML using DD4hep_PythonDetector to invoke the Python geometry.
examples/ClientTests/CMakeLists.txt	Install python/ example dir and add a new ClientTests regression test guarded by ROOT TPython support.
DDCore/src/python/PythonPlugin.cpp	Implement and register the DD4hep_PythonDetector factory; add env expansion + Python invocation glue.
DDCore/python/dd4hep_geo_plugin.py	New helper module for Python-based geometry plugins (cppyy helpers + type aliases).

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[Copilot]

py_str() uses snprintf and create_python_detector() uses std::uintptr_t, but this file doesn't include the headers that define them. Please add the appropriate includes (e.g. <cstdio> for snprintf and <cstdint> for std::uintptr_t) to avoid non-portable builds that may fail depending on transitive includes / compiler settings.

[Copilot]

expand_env() silently drops $VAR / ${VAR} tokens when the environment variable is not set, which can turn a configured pythonpath into an unintended absolute/relative path (e.g. ${DD4hepExamplesINSTALL}/... becomes /...) and make failures hard to diagnose. Consider either leaving the original token intact when getenv returns null, or throwing a DD4hep exception that clearly names the missing environment variable.

Suggested change

	if (val) result.append(val);
	if (!val) {
	except("PythonPlugin",
	"Missing environment variable '%s' while expanding: %s",
	var.c_str(), input.c_str());
	}
	result.append(val);

[Copilot]

module and function are required attributes for this factory, but the code reads them via attr<std::string> without a prior hasAttr check. If either is missing, the thrown exception will likely come from the XML layer and may not clearly indicate what's required for DD4hep_PythonDetector. Consider explicitly validating presence and raising a factory-specific error message (including the detector name/id and the missing attribute).