feat(rendering): add sky sphere background

[JeanPhilippeKernel]

Summary

Closes #803.
Part of #810.

• Adds SkySpherePass, an HDR fullscreen background pass with horizon, zenith, and ground gradients.
• Adds optional analytic sun-disc rendering driven by the explicitly selected primary directional light; the gradient remains valid when no light is selected or that actor is deleted.
• Integrates SkySphere as a normal render-graph callback, depth-tested only on background pixels and compatible with standard and reverse-Z depth conventions.
• Keeps SkySphere presentation-only: it immediately uses the permanent fallback IBL and never triggers HDRI decode, cubemap generation, or environment-lighting bakes.
• Fixes projective Mat4::Inverse() so camera ray reconstruction works for sky rendering rather than only affine transforms.
• Fixes cross-stage push-constant reflection to merge equal ranges instead of summing them, keeping the shared SkySphere range at the portable 80-byte size.
• Documents the fallback-IBL and sun-disc contract, and clears a deleted selected light reference from the hierarchy path.

Validation

• Built Obelisk and ZEngineTests on macOS Debug; new shaders compiled and were deployed with the editor.
• Passed 72 focused matrix, SkySphere, SkyEnvironment, and render-graph tests.
• Ran the repository pre-push clang-format gate successfully for engine, editor, and shader sources.
• Manually validated SkySphere, the directional-light sun disc, removal of the selected light, and viewport output.

Follow-up

• [Bug] Handle below-ground cameras in atmosphere views safely #822 tracks explicit below-ground camera classification and editor-safe atmosphere fallback. It is intentionally outside this SkySphere PR.