2.1.0: port of nianticlabs/spz v2.1.0

Initial commit: Swift port of nianticlabs/spz

Pure Swift implementation of the SPZ file format for 3D Gaussian splats.
Ported from https://github.com/nianticlabs/spz at v2.1.0, commit 2cf0be0

Features:
- Load/save SPZ files (gzip-compressed gaussian splat data)
- Full spherical harmonics support (degrees 0-3)
- Coordinate system conversion (RDF, RUB, etc.)
- No C/C++ dependencies (uses Foundation's Compression framework)