The datum plane detection in getDistanceType() only checked for
App::Plane (origin planes). This missed two other class hierarchies:
- PartDesign::Plane (inherits Part::Datum, NOT App::Plane)
- Part::Plane primitive referenced bare (no Face element)
Both produce empty element types (sub-name ends with ".") but failed
the isDerivedFrom<App::Plane>() check, falling through to
DistanceType::Other and the Planar fallback. This caused incorrect
constraint geometry, leading to conflicting/unsatisfiable constraints
and solver failures.
Add shape-based isDatumPlane/Line/Point helpers that cover all three
hierarchies by inspecting the actual OCCT geometry rather than relying
on class identity alone. Extend getDistanceType() to use these helpers
for all datum-vs-datum and datum-vs-element combinations.
Adds TestDatumClassification.py with tests for PartDesign::Plane,
Part::Plane (bare ref), and cross-hierarchy datum combinations.
