string-audit v0.2.1 — signed provenance (DSSE ed25519)

Each cached audit result's in-toto derivation is now a DSSE-signed envelope (ed25519): assembleEnvelope + ed25519Signer sign the PAE, the signature (keyid + sig) is attached and stored with the record. The signing identity is generated once and persisted in the room (gitignored). Closes the last provenance follow-up from #1 — derivations are signed, not just canonical.