[diegomazala]

Summary

Adds SPZ_NIANTIC_georeference (type 0x4E530001), an extension that records a similarity transform placing an SPZ asset's local frame into an Earth-centered geocentric CRS (e.g. EPSG:4978, WGS84 ECEF):

p_crs = scale * (q * p_local * q⁻¹) + origin

• origin (double[3], meters), rotation (double[4], unit quaternion), and scale (double, dimensionless uniform factor) carry the transform at full precision — independent of the 24-bit fixed-point quantization used for Gaussian positions themselves.
• scale is uniform (isotropic) only, matching the classic 7-parameter Helmert/similarity transform used for local-to-global CRS registration. It corrects for the scale ambiguity inherent to unreferenced reconstruction (e.g. monocular SfM), which is always isotropic under a calibrated-camera model — there is deliberately no anisotropic scale or shear field, since that would describe a different problem (a deformed local frame) than this extension addresses.
• crs_epsg names the target geocentric CRS; epoch (decimal year) and an optional WKT2 wkt string are provenance fields.
• This is a descriptor only — the library never applies the transform to Gaussian data. Readers that don't care about georeferencing can ignore it entirely; readers that do can apply the transform themselves in double precision at load time.
• Post-transform heights are ellipsoidal by construction. wkt records the source compound CRS (including vertical datum) as provenance — it is never authoritative for transforming data, since orthometric-vs-ellipsoidal height mixups silently shift world position by tens of meters.

Design notes / prior art

The split between a numeric transform (authoritative) and CRS metadata (descriptive) mirrors patterns in two existing geospatial formats:

• 3D Tiles (OGC standard) roots its scene graph with a transform — a 4×4 matrix mapping a tile's local frame into the tileset's global frame, commonly EPSG:4978 (ECEF). That matrix already supports scale as a first-class citizen (as does glTF's TRS node convention); this extension's scale field brings the same capability without requiring a full affine matrix.
• COPC (spec PDF, built on LAZ 1.4) carries CRS as a WKT string in a VLR/EVLR. This extension borrows the "optional WKT2 string" shape, but deliberately weakens it to provenance-only: COPC's WKT is the authoritative CRS, while here the numeric transform is authoritative and wkt is metadata about where it came from (compound CRS incl. vertical datum), since a reader must not re-derive a transform from it.

Testing

• extensions/cc/georeference-niantic.{h,cc} — payload struct, serialization, and load-time validation (ext_version, reserved crs_epsg=0, quaternion normalization, non-finite/non-positive scale, wkt_length bounds, oversized-wkt-on-write guard).
• tests/python/test_extensions.py — round-trips (with/without epoch and wkt), coexistence with SPZ_ADOBE_safe_orbit_camera, malformed-payload skip paths (including invalid scale: zero, negative, NaN, Inf), and Earth-radius origin precision.
• Bindings for Python (nanobind) and WASM (Emscripten/embind + TypeScript declarations) mirror the existing Adobe extension pattern.
• extensions/README.md documents the payload layout and validation rules; top-level README.md gets a georeferencing section; vendor ID 0x4E53 ("NS") is allocated in the vendor table.

Ran python -m pytest tests/python/ with SPZ_BUILD_EXTENSIONS=ON — 120 passed, 0 skipped, 0 failed.