Repo split from phalanx-fl¶

The corpus and measurement code was built inside phalanx-fl and moved here. It had grown to 80% of that repo's test suite while importing exactly one function from it, in a repo whose stated purpose is a Flower testbed.

The decisive argument was not tidiness but a policy conflict: phalanx's roadmap carries a recurring invariant to ride the latest Flower release, and a paper artifact needs the opposite. Those are contradictory release policies sharing one lockfile, already visible in Dependabot alerts pinned by flwr on a repo whose paper code never imported flwr.

phalanx.corpus became sphragis.corpus, plan B's three modules became sphragis.measure, and provenance_header was copied rather than imported.