Foundation Classes - remove PLib_Base and migrate to concrete polynomial types (...

Foundation Classes - remove PLib_Base and migrate to concrete polynomial types (#795)

- Delete PLib_Base.hxx / PLib_Base.cxx and remove from PLib/FILES.cmake.
- Convert PLib_JacobiPolynomial and PLib_HermitJacobi from polymorphic Handle-based types to direct value types:
  - Remove inheritance from PLib_Base and RTTI macros.
  - Replace DEFINE_STANDARD_HANDLE / Handle usage with plain objects/members.
  - Make internal PLib_JacobiPolynomial member const where appropriate.
- Make basis evaluation methods const (D0/D1/D2/D3 and internal D0123) and mark trivial accessors noexcept:
  - WorkDegree() and NivConstr() now noexcept.
  - Update signatures (const correctness) across implementation and headers.
- Update all call sites to new API and value semantics:
  - Tests: PLib_HermitJacobi_Test.cxx, PLib_JacobiPolynomial_Test.cxx updated to construct objects by value and call methods without '->'.
  - Modeling/approximation & FEM code: AdvApprox_ApproxAFunction, AdvApprox_SimpleApprox, AppDef_LinearCriteria, AppDef_Variational, FEmTool_Curve, FEmTool_ElementsOfRefMatrix, FEmTool_LinearFlexion/Jerk/Tension and related headers updated to accept/use PLib_HermitJacobi by reference/value and call new methods.
  - Replace Handle(...) constructions with stack/local objects and adapt calls (ToCoefficients, ReduceDegree, MaxError, AverageError, Points, Weights, etc.).
- Miscellaneous API adjustments to match new declarations (removed Standard_OVERRIDE annotations where not applicable).
- Keep behavior unchanged; this is an API/implementation refactor to remove legacy polymorphic base and improve const-correctness and performance.