Replace the hand-built toy elements with a single coupon produced by
`create_elements!`, matching how assembly pulls DOFs from handlers.
- SPDX header + one regression comparing flat vs structured extraction for T+u on Hex8.
New 218-line test file for extract_element_dofs functions:
- Tests single-field scalar extraction (Temperature)
- Tests single-field vector extraction (Displacement 2D/3D)
- Tests multi-field extraction (Temperature + Displacement)
- Tests non-contiguous DOF indices
- Tests both flat and structured extraction variants
- Validates zero-allocation for all extraction functions
- Tests NamedTuple return types with proper field names
Comprehensive test ensuring DOF extraction works correctly for
all field types and maintains zero-allocation performance.
Jukka Aho