- Procedural holographic Earth using simplex-noise continent shader
with scan lines, Fresnel rim glow, lat/lon grid overlay and
atmosphere shell (all pre-existing in main)
- Add gentle vertical float animation to earthGroup (±0.6 u, 28 s period)
- Add faint tether beam from Earth down to Nexus platform centre
Fixes#236
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>