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).

POP3Command.h

@@ -7,15 +7,34 @@
 namespace mail {
 
    class POP3Server;
-   
+
+   // RFC 1939 3: POP3 message numbers are 1-based positions in the
+   // maildrop's fixed-for-the-session ordering - session.mailbox's own
+   // ascending-by-UID message list, loaded once at PASS and never
+   // reloaded mid-session, already IS that fixed ordering, so message
+   // number N is simply session.mailbox.messages()[N-1]. Returns
+   // nullptr for an out-of-range number OR a number already marked
+   // \Deleted this session (RFC 1939 3: "the POP3 server MUST NOT" let
+   // a deleted message be referenced by any command).
+   MailFolder::Message *pop3FindMessage(POP3Session &session, int msgNum);
+
+   // Shared by RETR and TOP - RFC 1939 3's byte-stuffing rule (any line
+   // starting with "." gets an extra "." prepended, same transparency
+   // rule as SMTP DATA, RFC 5321 4.5.2) applies to both, and TOP is
+   // otherwise identical to RETR except it stops after maxBodyLines
+   // lines of body (headers and the header/body blank line always go
+   // out in full either way). maxBodyLines < 0 means unlimited (RETR's
+   // own call site).
+   void pop3SendMessage(POP3Session &session, const std::string &content, int maxBodyLines);
+
    class POP3Command : public core::Command {
-      
+
     public:
       virtual int processCommand(coreutils::ZString &request, core::TCPSession &session) override;
       virtual int processCommand(coreutils::ZString request, POP3Session &session, POP3Server &server);
-      
+
    };
-   
+
 }
 
 #endif