Add HDR OpenEXR cube face export

[andy83]

Summary

• export the existing HDR cube render target directly as six OpenEXR files
• add a Cube faces EXR (.zip) option to normal, batch, per-layer, and composite exports
• preserve existing PNG and equirectangular HDR/EXR export paths and canonical face naming

Implementation

The cubemap is already baked into a HalfFloatType render target. Each face is read into a Uint16Array, wrapped in a Three.js DataTexture, and encoded with EXRExporter. No 8-bit or PNG intermediate is used.

Faces are processed sequentially to keep memory usage manageable for 4096×4096 exports.

Verification

• npm test: 17 files, 135 tests passed
• npm run build: passed
• npm run lint: passed with one pre-existing react(only-export-components) warning
• all six 4096×4096 EXRs verified as half-float with HDR values above 1.0
• PNG/EXR orientation compared pixel-for-pixel across all faces
• batch and per-layer/composite packaging verified