Implement 3D Cosserat Media

[lsawade]

Adds a 3D micropolar (Cosserat) elastic medium with 6 DOFs (3 displacement + 3 rotation) throughout the mesher, mesh reader, solver infrastructure, and physics kernels. Uses medium_tag::elastic_spin + property_tag::isotropic_cosserat as the tag pair. No absorbing boundary conditions in this initial implementation.

Decisions

• Medium tag: elastic_spin (existing enum, new macro)
• DOFs: 6 — 3 displacement $(u_x, u_y, u_z)$ + 3 rotation $(\varphi_x, \varphi_y, \varphi_z)$
• Mesher domain ID: IDOMAIN_COSSERAT = 4
• Stacey absorbing boundaries: deferred to a follow-up PR
• Material parameters: identical to 2D — $(\rho, \kappa, \mu, \nu, j, \lambda_c, \mu_c, \nu_c)$