Rust migration Phase 1 — implements real functionality in the previously
stubbed mailer-core and mailer-security crates (38 passing tests).
mailer-core: Email/EmailAddress/Attachment types, RFC 5322 MIME builder,
email format validation with scoring, bounce detection (14 types, 40+
regex patterns), DSN status parsing, retry delay calculation.
mailer-security: DKIM signing (RSA-SHA256) and verification, SPF checking,
DMARC verification with public suffix list, DNSBL IP reputation checking
(10 default servers, parallel queries), all powered by mail-auth 0.7.
mailer-bin: Full CLI with validate/bounce/check-ip/verify-email/dkim-sign
subcommands plus --management mode for smartrust JSON-over-stdin/stdout IPC.
