[CanReader]

Moves SleakCraft onto the engine's per-format stems and lighting override (SleakEngine#233), so the BSL lighting work can live in game shaders.

• The voxel format registers gbuffer_voxel as its GBuffer fragment stem; it writes RT3 with the voxel shading model (craft/shading_models.glsl).
• Game::Initialize sets lighting_voxel as the deferred lighting shader. Voxel pixels go through the voxel path; everything else (block effects, any engine material) goes through the engine's default lighting.
• OpenGL gets matching _gl programs per pass; the old voxel-layout gbuffer_gl.vert becomes gbuffer_voxel_gl.vert, so the engine's default-layout GBuffer program is the global one again and block effects draw correctly on GL.

No visible change yet: Vulkan and OpenGL captures of the four fixed views are byte-identical. A throwaway patch painting the voxel shading model magenta proved RT3 reaches the lighting pass on both backends. The voxel lighting is a straight port of today's lighting for now; the BSL composition replaces it in the next change.

The Engine pin moves to the engine merge commit once SleakEngine#233 lands.