fix: Ed25519 grading, STARTTLS validation, and license detection

[dyaa]

• Return 256 bits for Ed25519 keys instead of 0, preventing incorrect grade downgrades
• Validate STARTTLS responses for SMTP, IMAP, POP3, FTP, XMPP, and Sieve instead of blindly proceeding on rejection
• Fix LICENSE wording to match canonical Apache 2.0 template so pkg.go.dev detects it correctly
• Document HSTS InsecureSkipVerify tradeoff, Valid field semantics, ctx parameter intent, and unencrypted CA key