`hse_fill` silently skips `+Z` HSE boundary handling on GPU builds

Summary

hse_fill() has explicit +Z HSE handling marked as unimplemented, but the error is guarded with #ifndef AMREX_USE_GPU. On GPU builds this path does nothing and returns silently, leaving +Z external HSE boundaries unfilled.

Location

• Source/problems/hse_fill.cpp:1017
• Source/problems/hse_fill.cpp:1019

Problem Details

Current code:

if (zr_ext_bc_type == EXT_HSE) {
#ifndef AMREX_USE_GPU
    amrex::Error("ERROR: HSE boundaries not implemented for +Z");
#endif
}

In GPU builds, no error is raised even though functionality is absent.

Impact

• Incorrect or uninitialized +Z ghost state for HSE external boundaries on GPU runs.
• Hard-to-diagnose physics errors because the failure is silent.

Suggested Patch

Fail fast in all builds until +Z HSE is implemented:

diff --git a/Source/problems/hse_fill.cpp b/Source/problems/hse_fill.cpp
--- a/Source/problems/hse_fill.cpp
+++ b/Source/problems/hse_fill.cpp
@@
         if (zr_ext_bc_type == EXT_HSE) {
-#ifndef AMREX_USE_GPU
             amrex::Error("ERROR: HSE boundaries not implemented for +Z");
-#endif  
         }

Prepared by Codex

Open Source/problems/hse_fill.cpp around lines 1017-1019 and inspect the +Z EXT_HSE branch and its AMREX_USE_GPU guard. Remove the GPU-only silence so unsupported +Z HSE handling fails consistently, then verify a GPU build or run that exercises +Z HSE boundaries and confirms an error is raised.