Make SatAdj unit tests hardware portable

[pressel]

Summary

This PR makes the Saturation Adjustment (SatAdj) unit tests hardware portable through AMReX and improves the inline documentation around the SatAdj test suite.

This is a testing and documentation-only update. It does not change the SatAdj physical formulation or production SatAdj physics behavior.

Changes

• Refactors the SatAdj MultiFab unit tests so they run through an AMReX-portable path rather than being skipped in GPU builds.
• Adds a portable kernel-path test that calls SatAdj::AdjustSatAdjCell from inside amrex::ParallelFor.
• Compares the kernel-path result against the existing host scalar reference.
• Keeps GTest assertions on the host and uses AMReX setup/checking paths for CPU and GPU portability.
• Adds targeted inline comments documenting:
  • the SatAdj test contract,
  • key physical invariants,
  • pressure-unit expectations,
  • the subtle evaporation-then-recondensation branch,
  • the SHOC no-op motivation,
  • why the kernel-path test exists,
  • the CUDA/GTest device-lambda placement constraint.
• Keeps the existing scalar thermodynamics and cell-property tests intact.

Motivation

The previous SatAdj unit tests covered the scalar physics well, but the MultiFab public-flow tests skipped GPU builds. Production SatAdj executes AdjustSatAdjCell inside an AMReX ParallelFor, so the tests should exercise that hardware-portable path as well.

This PR closes that gap without adding a separate GPU-only implementation of the tests. The same AMReX-portable test logic is used across backends.

What this does not change

• No change to the SatAdj physical formulation.
• No change to production SatAdj algorithm behavior.
• No new regression tests.
• No new gold files.
• No new runtime dependencies.