Lift the nix2container reproducible=false layer-chain helper (duplicated
verbatim across ii-agent, ii-researcher, temporal-based-ci, mempalace image,
ComfyUI) into parity.lib.foldImageLayers so the rationale lives in one place.
Add nixfmt-rfc-style formatter and a foldImageLayers contract probe to the
smoke check. Bump fleet-pins input to current HEAD.
Oleks