New file: test/domains/continuum/test_validation_hex8.jl
Tests for:
- Hex8 element stiffness matrix
- Comparison against reference solution
- Validates assembly with hexahedral elements
Validation:
- Creates single Hex8 cube element
- Assembles element stiffness matrix
- Compares against analytical or reference FEM solution
- Checks matrix symmetry and positive definiteness
Ensures assembly works correctly for element types beyond Tet4,
validating generality of the cache architecture.