17 contracts cite a since release that was never tagged (unmeasured in the provenance ledger)

[O6lvl4]

proofs/contract-provenance.toml (#20) classifies 17 contracts as unmeasured because their since names a release that no implementation tag carries:

since	contracts
0.34.4	C-160
0.37.1	C-191, C-192, C-193
0.38.1	C-194, C-195
0.53.7	C-215
0.54.1	C-216
0.56.1	C-220, C-221, C-222, C-223, C-224
0.56.2	C-225, C-226, C-227, C-228

since is defined as "the release in which the behaviour became normative — the first version a user can rely on it" (ledger header). A version that was never released cannot be that. For each: determine the first TAGGED release that carries the behaviour (the next tag after the since date, checked against the implementation's history) and correct since; or, if the version was tagged under another name, record it. Then python3 scripts/check-contract-provenance.py --write --impl-root <almide> — the rows leave unmeasured and the README provenance block updates. No normative change (S-editorial): the statements and evidence are untouched, only the release metadata.