[lohedges]

boresch_search only checked the two Boresch angles θA and θB for collinearity. It now also samples the bonded r1-r2-r3 and l1-l2-l3 anchor angles, and rejects candidates where either has a trajectory mean ≥ 160°, since the φA or φC dihedral is then undefined (e.g. a ligand triplet through a nitrile or alkyne). Both the rxrx and aldeghi protocols use the same cutoff. It sits well above any sp2/sp3 bond angle, so results don't change for inputs without an sp centre, and well below the ~170°+ mean of a thermally bent linear angle. Each anchor angle is measured once per unique triplet per frame, and RXRX gives a separate error message when only this check rejects every candidate.

Closes #513. Debugged with aid of Claude Code (Opus 5.5).

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): [y]
• I confirm that I have added a changelog entry to the changelog (we will add a link to this PR as part of the review): [y]
• I confirm that I have permission to release this code under the MIT license (the Python API) or the LGPLv3+ license (everything else), as described in the LICENSE file: [y]