Dependency security exceptions

Security advisories are denied by default. An exception is permitted only when the affected operation is unreachable, the boundary is enforced in code and a removal condition is recorded here.

RUSTSEC-2023-0071 (rsa)

• Introduced by: optional async-opcua support in aether-io.
• Risk: the upstream RSA implementation may leak private-key information through a timing side channel.
• Runtime boundary: aether-io accepts only anonymous OPC UA sessions with SecurityPolicy::None and MessageSecurityMode::None. Signing, encryption, username/password authentication and sample keypair generation are rejected before an OPC UA client is constructed.
• Default exposure: none. The opcua Cargo feature is disabled by default.
• Compensating control: use a trusted, isolated field network or a separately audited OPC UA bridge whenever transport security or authentication is needed.
• Audit policy: both deny.toml and the CI/local cargo audit invocation ignore only RUSTSEC-2023-0071; every other advisory remains denied.
• Removal condition: remove the cargo-deny and cargo-audit exceptions plus the temporary runtime restriction once async-opcua no longer resolves to an affected rsa release; then add authenticated and encrypted integration tests before re-enabling those modes.
• Review owner: maintainers; review on every async-opcua update and at least once per release.

This exception preserves protocol interoperability without claiming that the affected cryptographic operations are safe.