spirv-val layout bug

spirv-val flat out doesn't support GL_EXT_scalar_block_layout.

```cpp
#version 450
#extension GL_EXT_scalar_block_layout : enable

struct inner_t {
float x, y;
};

layout(binding=0, scalar)
uniform Uniform {
inner_t a, b;
} uniforms;

layout(location=0)
out vec4 color;

void main() {
color.x = uniforms.a.x;
}
```
error: line 34: Structure id 11 decorated as Block for variable in Uniform storage class must follow standard uniform buffer layout rules: member 1 at offset 8 is not aligned to 16
%Uniform = OpTypeStruct %inner_t %inner_t

spirv-val is insisting on 16 byte alignment for all struct members. glslc is emitting struct members aligned to their largest aligned type, which layout(scalar) is used.

Use the provided GLSL example as the reproducer and inspect spirv-val's handling of Uniform block layout rules. Compare its result with glslc's scalar-layout output; done means the example no longer reports the 16-byte alignment error while standard layout validation remains intact.

Written by the indexing model from the issue text.