Pylith poroelasticity test

[rwalkerlewis]

Add a fullscale poroelasticity test with prescribed nonzero fault slip to verify coupled solid-fluid behavior.

This new test case simulates a 0.5 m left-lateral slip on a vertical fault in a poroelastic medium, with drained boundaries for pore pressure dissipation and fixed far-field displacement boundaries. It verifies PyLith's ability to handle coupled poroelastic physics with a nontrivial fault slip scenario and the resulting pore pressure evolution.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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