…ng spec (#268)

The KHR_gaussian_splatting spec requires KHR_gaussian_splatting:SCALE to be
linear and non-negative, but the GLB writer stored the raw log-space scale
values (the internal/PLY convention). Spec-conformant viewers therefore failed
to visualize the exported splats.

Apply Math.exp() to the scale components on export, mirroring the sigmoid
activation already applied to opacity. exp() is always positive, so the
non-negative requirement holds by construction. This matches how scale is
interpreted elsewhere (the AABB/renderer path exponentiates the stored
log-space scale).

Add a regression test that parses the exported GLB and asserts SCALE equals
exp(input) and OPACITY equals sigmoid(input).

Fixes #267.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>