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).

tests/smtptest.py

@@ -0,0 +1,20 @@
+import smtplib
+import sys
+import time
+
+def send(subject, body, from_addr="test@barant.com", to_addr="barant@barant.com"):
+    msg = f"From: {from_addr}\r\nTo: {to_addr}\r\nSubject: {subject}\r\n\r\n{body}\r\n"
+    s = smtplib.SMTP("127.0.0.1", 25, timeout=15)
+    s.set_debuglevel(0)
+    s.ehlo("test-client.example.com")
+    s.sendmail(from_addr, [to_addr], msg)
+    s.quit()
+
+send("Auth stack test - normal mail", "This is an ordinary test message.")
+print("sent normal message")
+
+time.sleep(1)
+
+GTUBE = "XJS*C4JDBQADN1.NSBN3*2IDNEN*GTUBE-STANDARD-ANTI-UBE-TEST-EMAIL*C.34X"
+send("Auth stack test - spam probe", GTUBE)
+print("sent GTUBE spam-probe message")