Add real IMAP4rev1/POP3 implementations, DKIM/DMARC/SPF, SpamAssassin delivery hook

IMAP (RFC 3501): LOGIN through UID, previously all stubs except
CAPABILITY/LOGOUT/a partial APPEND. Multi-folder support with per-folder
UID/flag indexing (new MailFolder class, backed by .uidvalidity/.uidnext/
.imapindex sidecar files), ENVELOPE support (new MessageHeaders class:
header parsing, address-list parsing, IMAP quoting), and shared
cursor-based argument parsing (new IMAPArgs class: sequence sets, flag
lists, dates, folder-name normalization, and path-traversal protection
via isValidFolderName - a bare ".." folder name would otherwise resolve
into a sibling user's mailbox directory).

POP3 (RFC 1939): USER through QUIT, previously only CAPA. Added UIDL.
POP3 handlers use cursor-based parsing throughout rather than indexed
access - ZString's copy constructor doesn't copy its `list` field, and
POP3's dispatch passes the request by value, so indexed access into a
copy was silently reading nothing.

SMTP: DMARC evaluation added to the DATA-terminator path (DKIM
verification + alignment/policy, Authentication-Results header,
conditional rejection matching RFC 7489 6.3's p=reject + sampled pct=).
AUTH LOGIN now handles RFC 4954's optional inline initial response
(e.g. Python smtplib's "AUTH LOGIN <base64>" in one line) instead of
only the exact three-line form. Outbound relay now forces IPv4
(AF_INET) instead of AF_UNSPEC - this server's IPv4 has a correct PTR
record and SPF coverage, its IPv6 doesn't, and Gmail was rejecting
outbound mail with 550 5.7.25 as a result.

Delivery pipeline: local delivery now runs the message through
SpamAssassin (new SpamAssassinFilter class, speaking spamd's PROCESS
protocol directly over a raw socket - fails open/delivers unscanned if
spamd is unreachable) instead of a plain hardlink, since scanning can
tag/modify content.

BARANTMailConfig: extracted config loading into its own class.

compile: (see individual file changes - build script adjustments to
match the above).

Deliberately excluded from this commit: barantmail.log/
barantmail-service.log (operational logs, not source - barantmail.log
in particular was the runaway 55GB/12.9GB file from a busy-loop bug
fixed separately in ServerCore) and barantmail.cfg (this server's own
live deployment config, matching how APIServer's equivalent
config also stays untracked).

MessageHeaders.h

@@ -0,0 +1,54 @@
+#ifndef __MessageHeaders_h__
+# define __MessageHeaders_h__
+
+#include <string>
+#include <vector>
+
+namespace mail {
+
+   ///
+   /// Text-only header parsing shared by FETCH's ENVELOPE/BODY[HEADER...]
+   /// data items and SEARCH's FROM/TO/CC/SUBJECT criteria. Deliberately
+   /// does NOT parse MIME structure (no multipart, no Content-Type
+   /// tree) - see MailFolder.h's own doc comment on the same scoping
+   /// decision (BODYSTRUCTURE is out of scope; ENVELOPE only needs
+   /// header text).
+   ///
+
+   struct Address {
+      std::string name;
+      std::string mailbox;
+      std::string host;
+   };
+
+   // Splits a raw message into its header block and body, at the first
+   // blank line. header does NOT include the trailing blank line.
+   void splitMessage(const std::string &message, std::string &headerBlock, std::string &bodyBlock);
+
+   // Unfolds continuation lines (RFC 5322 3.2.2 - a line starting with
+   // SP/TAB continues the previous header) and returns every header as
+   // (name, value) in on-the-wire order. Value has the folding
+   // whitespace collapsed to a single space and is otherwise verbatim.
+   std::vector<std::pair<std::string, std::string>> parseHeaderLines(const std::string &headerBlock);
+
+   // First header matching name (case-insensitive), or "" if absent.
+   std::string getHeader(const std::vector<std::pair<std::string, std::string>> &headers, const std::string &name);
+
+   // "Name <user@host>, user2@host2" -> parsed addresses. Splits on
+   // top-level commas only (respecting "..." and <...>) - a simplified,
+   // not fully RFC 5322-compliant parser (no comments, no obsolete
+   // routing syntax), sufficient for real-world From/To/Cc header text.
+   std::vector<Address> parseAddressList(const std::string &headerValue);
+
+   // RFC 3501 7.4.2 ENVELOPE structure, built entirely from parsed
+   // headers - "sender"/"reply-to" default to "from" when their own
+   // header is absent, per the same RFC section's own convention.
+   std::string buildEnvelope(const std::string &message);
+
+   // nstring per RFC 3501 4.3 - a quoted string with backslash/quote
+   // escaped, or the literal NIL for an empty value.
+   std::string imapQuoteOrNil(const std::string &value);
+
+}
+
+#endif