Final mesh assembly and validation.
Output follows the established conventions exactly:
- elements 1..10 are the vertex elements "k 15 2 0 k k" on nodes 1..10
(front center, 4 front quarter points, back center, 4 back quarter pts)
- physical tags: Volume_1..nV, Tape_1..nT, DomainBoundary, DomainFront,
DomainBack (in this order); gap strip elements are dropped
- one $Periodic dim-2 link DomainBack -> DomainFront with all cap node
pairs (BELFEM derives the affine map from the vertex triples; the pairs
are kept for checking)