mailrs-dkim 3.0.0

RFC 6376 DKIM signature verifier + signer. Parses DKIM-Signature headers, applies simple/relaxed canonicalization, verifies rsa-sha256 / ed25519-sha256 against the selector TXT record. RSA primitive backed by aws-lc-rs for ~3× faster sign vs pure-Rust. Pluggable DNS via DkimResolver trait.