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-security/src/error.rs

@@ -0,0 +1,31 @@
+use thiserror::Error;
+
+/// Security-related error types.
+#[derive(Debug, Error)]
+pub enum SecurityError {
+    #[error("DKIM error: {0}")]
+    Dkim(String),
+
+    #[error("SPF error: {0}")]
+    Spf(String),
+
+    #[error("DMARC error: {0}")]
+    Dmarc(String),
+
+    #[error("DNS resolution error: {0}")]
+    Dns(String),
+
+    #[error("key error: {0}")]
+    Key(String),
+
+    #[error("IP reputation error: {0}")]
+    IpReputation(String),
+
+    #[error("parse error: {0}")]
+    Parse(String),
+
+    #[error("IO error: {0}")]
+    Io(#[from] std::io::Error),
+}
+
+pub type Result<T> = std::result::Result<T, SecurityError>;