Water plugin's RVT/ISR shader permutations fail PSO creation with instanced stereo (D3D12 SM6)

With Instanced Stereo Rendering enabled and the Water plugin enabled (default), PSO precaching speculatively compiles WorldGridMaterial × FWaterVertexFactory* permutations for the Runtime Virtual Texture pass. These PSO creations fail at D3D12 with E_INVALIDARG. The failures are non-fatal (precached PSOs that never get drawn), but the D3D12 debug layer logs them continuously.

Removing the Water plugin eliminates the errors entirely. No water assets exist in the level — this is purely the precacher walking the plugin's registered VF permutations against materials in the cook set.

1. Create a new UE 5.8 Preview VR project (or any project with Instanced Stereo Rendering enabled).
2. Confirm the Water plugin is enabled (default).
3. Cook + run a Development D3D12 SM6 build with -d3ddebug and -emulatestereo.
4. D3D12 debug layer logs InputLayout failures, and LogRHI reports failed PSO creation for the shader names above.