feat(import): cook HDR and EXR environment maps

[JeanPhilippeKernel]

Summary

• cook Radiance HDR and flat, single-part EXR equirectangular sources into UUID-keyed .zenvmap artifacts
• add TinyEXR decoding and strict artifact/source validation before runtime upload
• preserve canonical cubemap orientation and invalidate in-flight HDRI bakes after source reimports

Validation

• cmake --build --preset Darwin_arm64_Debug --target ZEngineTests --parallel 8
• HDRI/EXR importer and artifact tests (11 passed)
• sky environment regressions (26 passed)