Function inject_qa_stub 

pub(crate) fn inject_qa_stub(src: &str) -> String

_qa/_qb/…/_qh are MD2 runtime-defined aliases that pack the 32 q1..q32 evaluator channels into 8 × float4 (_qa = float4(q1, q2, q3, q4), _qb = float4(q5, …), etc.). User comp shaders read them directly, typically via mul(uv, float2x2(_qb)) for kaleidoscope rotation.

The standalone HLSL parser never sees their declarations because they live in the host-provided prelude — naga rejects with no definition in scope for identifier: '_qb'. We stub the full _qa..._qh family — many presets reference _qb and a long tail of the others.

For each referenced identifier we:

• prepend float4 _qX = float4(0.0, 0.0, 0.0, 0.0); at the start of shader_body { ... } so any later read picks up a defined value, and
• expand every float2x2(_qX) call into the four-scalar constructor form float2x2(_qX.x, _qX.y, _qX.z, _qX.w) so it survives replace_types into a valid mat2x2<f32>(...) call (WGSL refuses the single-vec4 form).

If no _qX identifier is referenced, returns the input unchanged.