fix(core): Suppress unused variable warnings in halo_pattern.hpp

Add [[maybe_unused]] attributes to global_world and grid variables that
are not currently used but may be needed for future features. Fixes
compilation warnings in Release builds where the compiler is more
aggressive about detecting unused variables.

Author: ahojukka5

include/openpfc/core/halo_pattern.hpp

@@ -197,8 +197,9 @@ core::SparseVector<BackendTag, size_t>
 create_recv_halo(const decomposition::Decomposition &decomp, int rank,
                  const Int3 &direction, int halo_width) {
   const auto &local_world = decomposition::get_subworld(decomp, rank);
-  const auto &global_world = decomposition::get_global_world(decomp);
-  const auto &grid = decomposition::get_grid(decomp);
+  [[maybe_unused]] const auto &global_world =
+      decomposition::get_global_world(decomp);
+  [[maybe_unused]] const auto &grid = decomposition::get_grid(decomp);
 
   auto local_size = world::get_size(local_world);
   auto local_lower = world::get_lower(local_world);