update main to upstream branch

… unit tests for refactored basis functions

  ReferenceNodeLayout::serendipity_node_coords owner

…ons are used

- include the basis family in the documented basis identity (topology, order, family)
- replace the order() normalization wording with a plain description and example
- add cross-reference links to the module entities in the group documentation
- add topology and family examples to the basis object contract
- refer to shape function values in the BasisFunction class description

Add an explanatory sentence and a concrete example to each derived basis
exception, drawn from its real throw sites, so the specific types guide
developers on when to raise or expect them.

…ation history

Name the orders and elements that use equispaced nodes directly, instead of
referring to what previous element layouts kept, so the note stays accurate as
the implementation evolves.

create() and create_default_for() now return std::unique_ptr<BasisFunction>,
which has simpler ownership semantics and is cheaper than shared_ptr. The single
caching consumer keeps a shared_ptr cache; the returned unique_ptr converts to it
on insertion. All FE/Basis unit tests pass.

…erloads in the base

The span *_to evaluators are now the primitives a concrete basis implements:
evaluate_values_to is pure virtual, evaluate_gradients_to/evaluate_hessians_to
default to reporting not-implemented, and a protected evaluate_all_to provides the
single-pass combined evaluation. The std::vector overloads (evaluate_values,
evaluate_gradients, evaluate_hessians, evaluate_all) are implemented once on the
base class, so LagrangeBasis and SerendipityBasis no longer duplicate those
wrappers.

This inverts the previous arrangement where the vector form was the primitive and
the base supplied an allocate-and-copy span fallback. A minimal basis now
overrides the span primitive instead of the vector form. The BasisFunction test
helpers are updated accordingly, and the former fallback test is rewritten to
verify the base vector overloads forward to the span primitives. All FE/Basis
unit tests pass.

…tation

- note that the vector evaluators use an output argument so callers can reuse a
  buffer across evaluations rather than allocating per call
- document that the BasisTraits classifiers return -1 / Unknown as sentinels that
  callers validate into exceptions (the constexpr noexcept helpers cannot throw)
- keep the BasisRequest::topology field-ordering note out of the rendered docs

…roup

Replace the @cond INTERNAL exclusion with a documented internal group whose
@warning states the declarations are internal (use basis_factory and
BasisFunction::nodes() instead) and may change. Core developers now get the
rendered documentation while model-level callers are clearly steered away.

… Vector

Document that FE/Math/Vector.h is a fixed-size, compile-time-length element-level
vector in svmp::FE::math, distinct from and not a replacement for the legacy
dynamic global ::Vector in solver/Vector.h.

…nd ElementType bands

- explain that the Mesh library is an optional external module and FE builds
  standalone with fallback types when it is absent
- record why GlobalIndex stays a plain alias (raw PETSc/Trilinos interop) and that
  DofIndex is the strong wrapper
- document that the explicit ElementType values are intentional bands with a
  uint8_t Unknown sentinel

…e_label

Add Doxygen to the dense linear-algebra functions and the DenseLUSolver type, and
rename the label argument (and the DenseLUSolver member) to error_message_label so
its role as an error-message prefix is self-documenting. All FE math unit tests pass.

Add a default case that raises BasisElementCompatibilityException for a solver
element type with no FE mapping, so a missing mapping fails loudly instead of
relying on the unhandled-enum compiler warning. The deliberate NA/PNT/NRB cases
still return std::nullopt.

MathJax 2 was pinned as a default when the FE documentation was added and is now
end-of-life. Move to the maintained MathJax 3 (chtml output, mathjax@3 CDN, ams
extension); doxygen emits the v3 bootstrap and the AMS math used in the FE docs is
supported.

…mbers

Generating HTML (not just parsing) surfaced reference/doc issues the no-output
checks missed:
- qualify the @ref class/struct/enum links in the Basis group doc so they resolve
- use plain text for the basis_factory namespace mention (no resolvable @ref)
- avoid the ::Vector explicit-link request in the Math vector doc
- add @return tags and document the NodeOrderingConventions entities that became
  visible when the @cond INTERNAL exclusion was removed
Net FE Doxygen warnings drop from 47 to 19 (the rest pre-existing).

…assertion helpers

- add svmp::NotImplementedException and svmp::IndexOutOfRangeException (CoreException-derived)
- default the ExceptionT template parameter of not_implemented() and check_index()
  to those types, so they can be used without naming an exception while existing
  explicit callers are unaffected
- document the helpers, including that check vs check_arg are the same check named
  for intent and that throw_if is the logical inverse of check, and when to use
  each not_implemented overload
Note: svmp::FE::NotImplementedException stays for FE code needing FEException
ancestry. All FE/exception-helper unit tests pass.

…ption infrastructure