Function write_pair_proof_to 

pub fn write_pair_proof_to(path: &Path, proof: &PairProof) -> Result<()>

Atomically write proof to path through crate::sidecar::write_atomic: unique temp sibling, explicit mode, fsync, rename, directory fsync. The fsync is the point — this record is the only thing bounding a bind storm across a reboot, so it must survive a power cut mid-write.