Initializing hiding rigidity from the extracted Pauli-X family #
The fine Pauli-X replacement estimate is first converted to cross-party
consistency using its exact EPR mirror. Projective coarse-graining therefore
costs nothing. The actual Pauli-X/first-hiding edge then identifies the first
hiding family, with error 2 deltaX + 4 |E| epsilon on a full EPR-plus-auxiliary
state. No first-hiding rigidity hypothesis is used.
Stated in a bipartite model (Phase 4 of planning/mipco-track.md): the EPR-plus-auxiliary state is
the register model Ξ.reg (ι → F) of a normalized auxiliary model, and the honest families enter
as smulKron 1 _.
A cross-party extracted Pauli-X guarantee initializes the actual hiding chain. Only the Pauli-X measurement needs projectivity at this step.
The usual same-party extracted Pauli-X replacement estimate suffices: the ideal X projectors are exact mirrors on the EPR register, independently of the auxiliary state.