Closed-form Heikkinen method for ECEF conversion (centimetre accuracy,
no iteration). ZYX aerospace-convention Euler → quaternion. Both needed
by upcoming VPAIRAdapter rewrite; reusable for other datasets shipping
ECEF or Euler poses (e.g. some MARS-LVIG releases).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Yuzviak