Add input checks to stagnant lid comparison plot

[ss2098]

This PR adds a preflight input check to scripts/stagnant_lid_adapt_compare_plot.py.

The script is a post-processing plot script and expects existing stagnant-lid checkpoint outputs. Previously, if the hard-coded checkpoint directories or files were missing, PETSc/HDF5 failed later with a low-level H5Fopen error.

This update checks for the required mesh and T checkpoint files before loading them and prints a clear message explaining which inputs are missing and that the user should run/copy the checkpoint outputs first or update U_DIR/A_DIR and U_STEM/A_STEM.

This does not change solver internals or checkpoint loading behavior when the required files exist.

Validation:

• python -m py_compile scripts/stagnant_lid_adapt_compare_plot.py
• pixi run python scripts/stagnant_lid_adapt_compare_plot.py

With missing local checkpoint files, the script now exits cleanly after listing the missing required inputs instead of failing with a PETSc/HDF5 traceback.

[ss2098]

The failing test_uw3 import-smoke check appears to fail during MPI/UCX initialization before this PR change is reached:

UCX ERROR mana_0 ... failed to create UD QP
MPI_Init_thread failed
MPIDI_UCX_init_worker Input/output error

This PR only adds preflight input checks to scripts/stagnant_lid_adapt_compare_plot.py.

Local validation passed with:

python -m py_compile scripts/stagnant_lid_adapt_compare_plot.py
pixi run python scripts/stagnant_lid_adapt_compare_plot.py

With missing local checkpoint inputs, the script now exits cleanly after listing the missing files instead of failing later with a PETSc/HDF5 traceback.

I do not have permission to rerun the upstream workflow, so a maintainer rerun may be needed.