Drive refinement through `create_structured_box_mesh` so tests stay aligned with
the dimensioned `Mesh{8,Hex8}` constructors, and keep a cheap analytic volume
sanity check for `compute_element_volume`.
- SPDX header; replace bespoke manual meshes with structured coupons + bisection counts.
New 166-line test file for mesh refinement operations:
- Tests single Hex8 refinement at 1, 2, and 3 levels
- Tests rectangular Hex8 refinement (longest edge bisection)
- Tests multiple element refinement
- Tests element set preservation during refinement
- Validates element and node count after refinement
- Validates refinement along longest dimension
Tests LongestEdgeBisection strategy for adaptive mesh refinement.
Jukka Aho