[Feature] Implement SkySphere background composition

[JeanPhilippeKernel]

Type

• Bug
• Feature
• Refactor / cleanup
• Task / chore

Overview

Ship the first visible sky mode: an analytic SkySphere background with optional sun disc and correct HDR composition.

Context

SkySphere is a presentation mode only. It uses fallback IBL rather than generating a physically accurate environment capture.

What needs to be done

• Add SkySpherePass as a RenderGraph graphics pass using the scene SkyConfig.
• Composite behind opaque geometry with correct depth, reverse-Z, HDR, and post-processing order.
• Implement horizon, zenith, ground, tint, intensity, and optional primary-sun-disc controls.
• Preserve valid fallback IBL while this mode is selected.

Resources

• ZEngine/docs/future-plan/sky-rendering.md sections 1, 6, and 10

Testing

Manually test empty and populated scenes, multiple viewports, resize, reverse-Z, disabled sun, and editor game-view output.

Acceptance criteria

• SkySphere is selectable per scene and produces a non-black valid background.
• Opaque and transparent scene composition is correct.
• The mode works without an HDRI asset or directional light.
• Lighting continues to use valid fallback IBL.
• Builds without warnings in Debug and Release.
• All relevant unit tests pass.
• No regression in existing functionality.

Estimated effort

2-3 days.