fix(rendering): handle below-ground atmosphere views (#822)

[JeanPhilippeKernel]

Summary

• classify atmosphere camera views and skip invalid or below-surface per-view integration
• use an orientation-independent linear-blue skybox fallback for those views
• add an opt-in editor camera constraint at the planet surface
• cover view classification and the camera constraint with tests

Closes #822

Validation

• macOS arm64 Debug build
• ZEngineTests --gtest_brief=1
• repository clang-format pre-push checks