v1.2.0
Additive throughout. backup.v1.json is untouched, ArtifactStream is untouched, and the signing fixtures still verify byte for byte, so a 1.1 connector and a 1.2 platform remain compatible in both directions. No new capability. Adds ArtifactEnvelope, KeyFingerprint, RecoveryProof, Sealing::isUsableBoxPublicKey(), and the backup.v2, backup-manifest.v2 and backup-progress.v1 schemas, with a byte-exact reference artifact that three implementations parse. 287 tests, PHPStan level 8 clean.