[EXAMPLE-1] Add at least one more 3D example

[lsawade]

Add acoustic elastic example

Problem: There is only 1 example under examples/dim3/ (homogeneous halfspace) compared to 4 under examples/dim2/. The 3D code path is severely under-exercised in the example suite.
Address plan:
Add examples/dim3/fluid-solid-interface/ or examples/dim3/marmousi3d/ mirroring a 2D equivalent.
Include a README.md, parameter YAML file, and mesh generation script.
Register the example as a CTest integration test so CI exercises the 3D path.
Files: examples/dim3// (new), integration test CMakeLists