remove 'unused step' warnings from kernels without pd

[pkienzle]

For models without polydispersity, the c compiler was returning warnings about an unused step parameter for every variant of the calculator (Iq, Iqxy, Imagnetic). This made it hard to see the actual error.

After the fix the following

2026-06-01 21:49:10,725 : ERROR : sas.qtgui.Utilities.ModelEditors.TabbedEditor.TabbedModelEditor (TabbedModelEditor.py:466) :: Error building model: compile failed.
cc -std=c99 -O2 -Wall -fPIC -shared /tmp/sas64_arm64_yuk_29A943E8_egi_n035.c -o ".../SasView/compiled_models/sas64_arm64_yuk_29A943E8.so" -lm
.../SasView/plugin_models/yuk.py:65:5: error: use of undeclared identifier 'broken'
   65 |     broken;
      |     ^~~~~~
./kernel_iq.c Iq:677:5: warning: variable 'step' set but not used [-Wunused-but-set-variable]
  677 | int step = pd_start;
      |     ^
./kernel_iq.c Iqxy:677:5: warning: variable 'step' set but not used [-Wunused-but-set-variable]
  677 | int step = pd_start;
      |     ^
./kernel_iq.c Imagnetic:677:5: warning: variable 'step' set but not used [-Wunused-but-set-variable]
  677 | int step = pd_start;
      |     ^
3 warnings and 1 error generated.

becomes

2026-06-01 21:53:07,905 : ERROR : sas.qtgui.Utilities.ModelEditors.TabbedEditor.TabbedModelEditor (TabbedModelEditor.py:466) :: Error building model: compile failed.
cc -std=c99 -O2 -Wall -fPIC -shared /tmp/sas64_arm64_yuk_756BDC0C_cb1sfuot.c -o ".../SasView/compiled_models/sas64_arm64_yuk_756BDC0C.so" -lm
.../SasView/plugin_models/yuk.py:65:5: error: use of undeclared identifier 'broken'
   65 |     broken;
      |     ^~~~~~
1 error generated.

[krzywon]

Code seems reasonable. Haven't tested.

[pkienzle]

CI runs C models with and without polydipersity. Since everything still passes it doesn't break anything.

Showing that the warning doesn't appear on your system is more complicated because compiler output is suppressed if there are no errors. I modified sasmodes/models/guinier.c with:

...
#warning "this is a warning"
#error "this is an error"

then ran the model under cpu and gpu respectively:

python -m sasmodels.compare -single! guinier
python -m sasmodels.compare -single guinier

In this branch on mac os I only got the above warning and error. On master I got the unused variable warning for -single! (cpu) but not for -single (gpu).