refactor: simplify hypoelastic source terms in m_hypoelastic.fpp

[sbryngelson]

Summary

Addresses #1343 (readability only — no correctness change).

The source term formula is $S^e_{ij} = \rho(l_{ik}\tau_{kj} + \tau_{ik}l^T_{kj} - \tau_{ij},\mathrm{tr}(\mathbf{D}) + 2G,D^d_{ij})$. Because $\mathbf{l}\tau + \tau\mathbf{l}^T$ is symmetric and doubles the off-diagonal contributions, the old code wrote e.g. τ_12*du_dy + τ_12*du_dy as two separate lines. The zero-sum pairs (e.g. + τ_12*du_dx - τ_12*du_dx in the τ_12 update) are algebraic cancellations from expanding $\mathbf{l}\tau + \tau\mathbf{l}^T$ before simplifying.

This PR collapses each duplicate pair to 2._wp * τ * dv and removes the zero-sum pairs, making the code match the formula directly. The change is purely cosmetic: no numerical values change.

Changes

src/simulation/m_hypoelastic.fpp, idir == 2 and idir == 3 blocks:

• Collapse x + x → 2._wp*x for the doubled off-diagonal rotation terms
• Drop +a - a zero-sum pairs in the τ_12, τ_13, τ_23 shear updates
• Simplify 2._wp*G*(1._wp/2._wp)*... → G*... and 2._wp*G*(1._wp/3._wp)*... → (2._wp/3._wp)*G*...

Net: 36 lines → 19 lines (−17 lines).

Test plan

• Hypoelastic regression tests pass (results must be bit-identical to master)