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Introduce OPENMM_HIP_PME_FAST_GRID_LEVEL as a HIP-only experimental
control for reducing automatically selected PME grid dimensions.  A
positive level repeatedly steps each grid dimension down to the previous
legal FFT size, while level 0 preserves the existing OpenMM behavior.

Keep explicit user-provided PME grid parameters unchanged so the feature
only affects automatic grid selection.  This is intended for performance
and accuracy exploration, not as a default tuning policy.