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)

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📥 Commits

Reviewing files that changed from the base of the PR and between d513442 and 8c1f7ef.

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• src/simulation/m_hypoelastic.fpp

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[sbryngelson]

Automated Code Review

Summary: No issues found. ✅

Performed term-by-term algebraic verification of every simplification in the idir==2 and idir==3 blocks:

• tau_xy*du_dy + tau_xy*du_dy → 2._wp*tau_xy*du_dy ✓
• 2._wp*G*(1._wp/3._wp) → (2._wp/3._wp)*G ✓
• 2._wp*G*(1._wp/2._wp) → G ✓
• All zero-sum cancelling pairs (+a - a) correctly dropped ✓
• Off-diagonal rotation terms correctly identified and collapsed ✓

All simplifications are algebraically exact. Precision conventions respected throughout (2._wp, (2._wp/3._wp), no d0 literals). No raw GPU pragmas introduced. Safe to merge.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.75%. Comparing base (d513442) to head (8c1f7ef).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1399   +/-   ##
=======================================
  Coverage   64.75%   64.75%           
=======================================
  Files          71       71           
  Lines       18721    18721           
  Branches     1551     1551           
=======================================
  Hits        12123    12123           
  Misses       5640     5640           
  Partials      958      958           

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