Restore PBAM and PBSAM integration

[sobolevnrm]

Summary

• propagate the FETK mc target requirements to the fetched PBAM and PBSAM libraries
• zero-initialize the C-to-C++ PBSAM input structures instead of passing indeterminate values
• always transfer the parser default for gridPts and transfer the missing PBSAM surface-file count

Validation

• clean macOS build with ENABLE_PBSAM=ON, ENABLE_PBAM=OFF
• clean macOS build with ENABLE_PBAM=ON, ENABLE_PBSAM=OFF
• clean macOS build with both options enabled
• PBAM energy/force example exits successfully
• PBSAM barnase/barstar calculation exits successfully when run with known-valid coarse-grained PQR fixtures

Remaining work

The checked-in pbsam-barn_bars example still supplies atomistic PQR files without embedded CEN spheres and does not include the commented-out MSMS mesh files. This draft intentionally does not change those scientific inputs. The example therefore still raises CouldNotReadException for an empty mesh path until we decide whether to ship validated coarse-grained PQRs, ship the corresponding meshes, or add explicit missing-surface validation.

Refs #298