[Task] Retire legacy SkyboxPass after sky parity

[JeanPhilippeKernel]

Type

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

Overview

Remove the legacy SkyboxPass only after the replacement sky system has proven presentation, fallback, and environment-lighting parity.

Context

The old pass remains a fallback implementation during staged delivery. Its removal must not leave scenes with no background or invalid IBL descriptors.

What needs to be done

• Establish feature and visual parity criteria for SkySphere, HDRI, and atmosphere modes.
• Migrate renderer registration and tests to the replacement system.
• Remove legacy shaders, pass registration, dead configuration, and obsolete documentation.
• Confirm project and scene loading work after the intentional SkyConfig schema break.

Resources

• ZEngine/docs/future-plan/sky-rendering.md sections 1 and 13

Testing

Run full renderer tests and manually verify existing sample scenes, editor viewport, game view, loading fallback, and post-processing output.

Acceptance criteria

• Replacement modes cover every supported legacy SkyboxPass use case.
• No renderer path references SkyboxPass or its obsolete assets.
• Editor and game views retain a valid background and IBL during loading or failure.
• Builds without warnings in Debug and Release.
• All relevant unit tests pass.
• No regression in existing functionality.

Estimated effort

1-2 days.