feat(rust): implement mailer-core and mailer-security crates with CLI

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.

rust/crates/mailer-core/src/lib.rs

@@ -1,9 +1,25 @@
 //! mailer-core: Email model, validation, and RFC 5322 primitives.
 
+pub mod bounce;
+pub mod email;
+pub mod error;
+pub mod mime;
+pub mod validation;
+
+// Re-exports for convenience
+pub use bounce::{
+    detect_bounce_type, extract_bounce_recipient, is_bounce_subject, BounceCategory,
+    BounceDetection, BounceType,
+};
+pub use email::{extract_email_address, Attachment, Email, EmailAddress, Priority};
+pub use error::{MailerError, Result};
+pub use mime::build_rfc822;
+pub use validation::{is_valid_email_format, validate_email, EmailValidationResult};
+
 /// Re-export mailparse for MIME parsing.
 pub use mailparse;
 
-/// Placeholder for email address validation and data types.
+/// Crate version.
 pub fn version() -> &'static str {
     env!("CARGO_PKG_VERSION")
 }