test(fft): Fix GridSpacing initializers in backend selection tests

Changed single-value initializers {N} to proper 3D format {N, N, N}
for GridSize, PhysicalOrigin, and GridSpacing in all backend
selection test cases. Single-value initializers only set the first
dimension, leaving y=0 and z=0, which fails coordinate system
validation requiring spacing > 0 in all dimensions.

Fixed in 6 test cases:
- FFTW backend selection: {8, 8, 8}
- FFTW forward/backward transform: {8, 8, 8}
- CUDA backend selection: {8, 8, 8}
- CUDA requires DataBuffer: {8, 8, 8}
- Timing functions work: {8, 8, 8}
- Size queries work: {16, 16, 16}

Also added .0 suffix to make Real3 type usage explicit.

Fixes "Spacing must be positive" test failures.

Author: ahojukka5

tests/unit/fft/test_fft_backend_selection.cpp

@@ -25,7 +25,8 @@ using namespace pfc;
 using json = nlohmann::json;
 
 TEST_CASE("FFT Backend - FFTW backend selection", "[fft][backend][unit]") {
-  auto world = world::create(GridSize({8}), PhysicalOrigin({8}), GridSpacing({8}));
+  auto world = world::create(GridSize({8, 8, 8}), PhysicalOrigin({8.0, 8.0, 8.0}),
+                             GridSpacing({8.0, 8.0, 8.0}));
   auto decomposition = decomposition::create(world, 1);
 
   // Create FFT with FFTW backend explicitly
@@ -38,7 +39,8 @@ TEST_CASE("FFT Backend - FFTW backend selection", "[fft][backend][unit]") {
 }
 
 TEST_CASE("FFT Backend - FFTW forward/backward transform", "[fft][backend][unit]") {
-  auto world = world::create(GridSize({8}), PhysicalOrigin({8}), GridSpacing({8}));
+  auto world = world::create(GridSize({8, 8, 8}), PhysicalOrigin({8.0, 8.0, 8.0}),
+                             GridSpacing({8.0, 8.0, 8.0}));
   auto decomposition = decomposition::create(world, 1);
   auto fft = fft::create_with_backend(decomposition, 0, fft::Backend::FFTW);
 
@@ -65,7 +67,8 @@ TEST_CASE("FFT Backend - FFTW forward/backward transform", "[fft][backend][unit]
 
 #if defined(OpenPFC_ENABLE_CUDA)
 TEST_CASE("FFT Backend - CUDA backend selection", "[fft][backend][cuda][unit]") {
-  auto world = world::create(GridSize({8}), PhysicalOrigin({8}), GridSpacing({8}));
+  auto world = world::create(GridSize({8, 8, 8}), PhysicalOrigin({8.0, 8.0, 8.0}),
+                             GridSpacing({8.0, 8.0, 8.0}));
   auto decomposition = decomposition::create(world, 1);
 
   // Create FFT with CUDA backend explicitly
@@ -78,7 +81,8 @@ TEST_CASE("FFT Backend - CUDA backend selection", "[fft][backend][cuda][unit]")
 }
 
 TEST_CASE("FFT Backend - CUDA requires DataBuffer", "[fft][backend][cuda][unit]") {
-  auto world = world::create(GridSize({8}), PhysicalOrigin({8}), GridSpacing({8}));
+  auto world = world::create(GridSize({8, 8, 8}), PhysicalOrigin({8.0, 8.0, 8.0}),
+                             GridSpacing({8.0, 8.0, 8.0}));
   auto decomposition = decomposition::create(world, 1);
   auto fft = fft::create_with_backend(decomposition, 0, fft::Backend::CUDA);
 
@@ -142,7 +146,8 @@ TEST_CASE("FFT Backend - CUDA backend throws if not compiled",
 #endif
 
 TEST_CASE("FFT Backend - timing functions work", "[fft][backend][unit]") {
-  auto world = world::create(GridSize({8}), PhysicalOrigin({8}), GridSpacing({8}));
+  auto world = world::create(GridSize({8, 8, 8}), PhysicalOrigin({8.0, 8.0, 8.0}),
+                             GridSpacing({8.0, 8.0, 8.0}));
   auto decomposition = decomposition::create(world, 1);
   auto fft = fft::create_with_backend(decomposition, 0, fft::Backend::FFTW);
 
@@ -169,7 +174,8 @@ TEST_CASE("FFT Backend - timing functions work", "[fft][backend][unit]") {
 TEST_CASE("FFT Backend - size queries work through interface",
           "[fft][backend][unit]") {
   auto world =
-      world::create(GridSize({16}), PhysicalOrigin({16}), GridSpacing({16}));
+      world::create(GridSize({16, 16, 16}), PhysicalOrigin({16.0, 16.0, 16.0}),
+                    GridSpacing({16.0, 16.0, 16.0}));
   auto decomposition = decomposition::create(world, 1);
   auto fft = fft::create_with_backend(decomposition, 0, fft::Backend::FFTW);