[spirv-val] Incorrect error message on local size

```
error: Local Size execution mode must not have a product of zero (X = 1048576, Y = 1048576, Z = 1048576)
```

I'm pretty sure `1048576^3 != 0`.

Start by locating the spirv-val validation check that emits the shown “Local Size execution mode” error. Trace how the X, Y, and Z values are combined, then add regression coverage showing that the reported values do not produce a zero product and verify the corrected diagnostic.

Written by the indexing model from the issue text.