Both DX backends ignore `VertexFormatHandle` entirely: the custom vertex format API is...

[CanReader]

src/Graphics/DirectX11/DirectX11Renderer.cpp:487; src/Graphics/DirectX12/DirectX12Renderer.cpp:1012;
src/Graphics/DirectX11/DirectX11Shader.hpp:57-64; src/Graphics/DirectX11/DirectX11Shader.cpp:97

grep -n "VertexFormat" src/Graphics/DirectX11/*.cpp src/Graphics/DirectX12/*.cpp returns zero hits.
Both backends hardcode the stride:

• DX11: UINT stride = sizeof(Sleak::Vertex); (:487)
• DX12: vbView.StrideInBytes = sizeof(Sleak::Vertex); (:1012)

GL (OpenGLRenderer.cpp:349-351) and Vulkan (VulkanRenderer.cpp:687, VulkanPipelines.cpp:741) both honor
the handle. Sleak::Vertex is 96 bytes (MeshData.hpp:14 states so in its own doc comment); SleakCraft's
VoxelVertex is 48.

It compounds at the input layout: DirectX11Shader.hpp:57-64 hardcodes DefaultLayout offsets
0/12/24/40/56, and DirectX11Shader.cpp:97 picks between exactly two fixed tables based only on
whether the VS declares BLENDINDICES. So flat_shader.hlsl: whose VS_INPUT (lines 8-14) is precisely
the 48-byte voxel layout with color@24 and uv@40: is fed color from byte 40 and UV from byte 56, at double
the correct stride.

Consequence: SleakCraft renders garbage geometry on -r d3d11 and -r d3d12. This is the engine's headline
custom-vertex-format abstraction (CLAUDE.md §3) silently absent on two of four backends.

Severity: corruption.

Found by a full audit of the graphics layer. Assessed severity: corruption.