[geometry] Preserve rotation axis signs at exact half turns

[Arthur031221]

Description

Users working with synthetic relative poses can load a different rotation from pairs JSON for an exact symmetric half turn.

In a noise-free reproduction with E = diag(0, 1, -1) and 12 fixed correspondences, estimateTransformStructureFromEssential returns an exact symmetric half-turn. Its JSON round trip changes two rotation entries from -1 to +1 before the fix. The fixed round trip preserves the rotation within 1e-12.

Features list

• Recover the axis signs used to serialize half-turn relative poses.
• Add deterministic log/exp round-trip tests for exact and near half turns.

Implementation remarks

The near-pi branch selects the largest diagonal entry of the symmetric matrix instead of using antisymmetric differences, which vanish at an exact half turn.

The exact-half-turn test fails with the original header and passes with the fix. All 24 near-half-turn cases pass with both headers. Validation covers the five geometry test executables, which passed twice.

[servantftransperfect]

LGTM. Thanks a lot.