Gate checkpoint reads by support, not family recognition

ParseFormat previously hard-failed on any unrecognized family, so a checkpoint written by a newer client surfaced as a malformed-format error and IsUnsupportedVersion returned false, suppressing the upgrade prompt.

Validate only the family-vN structure and let CanRead be the sole read gate. Unknown but well-formed families now parse and resolve to an unsupported-version error, so the upgrade path fires as intended.