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/pop3test.py

@@ -0,0 +1,63 @@
+import poplib
+import ssl
+
+ctx = ssl.create_default_context()
+ctx.check_hostname = False
+ctx.verify_mode = ssl.CERT_NONE
+
+USER = "barant@barant.com"
+PASS = "imaptest123"
+
+def step(name, fn):
+    try:
+        result = fn()
+        print(f"OK   {name}: {result}")
+        return result
+    except Exception as e:
+        print(f"FAIL {name}: {e}")
+        raise
+
+m = poplib.POP3_SSL("127.0.0.1", 995, context=ctx)
+print("welcome:", m.getwelcome())
+
+step("USER", lambda: m.user(USER))
+step("PASS", lambda: m.pass_(PASS))
+count, size = step("STAT", lambda: m.stat())
+print(f"  {count} messages, {size} octets")
+
+resp, items, octets = step("LIST", lambda: m.list())
+print(f"  {len(items)} items, sample: {items[0] if items else None}")
+
+if count > 0:
+    resp, lines, octets = step("RETR 1", lambda: m.retr(1))
+    print(f"  {len(lines)} lines, first: {lines[0] if lines else None}")
+
+    resp, uidls, octets = step("UIDL (all)", lambda: m.uidl())
+    print(f"  sample: {uidls[0] if uidls else None}")
+
+    resp = step("UIDL 1", lambda: m.uidl(1))
+    print(f"  {resp}")
+
+    resp, lines, octets = step("TOP 1 0", lambda: m.top(1, 0))
+    print(f"  {len(lines)} lines (should be 0 body lines)")
+
+    step("DELE 1", lambda: m.dele(1))
+    count2, size2 = step("STAT (after DELE)", lambda: m.stat())
+    print(f"  {count2} messages (should be {count - 1})")
+
+    step("RSET", lambda: m.rset())
+    count3, size3 = step("STAT (after RSET)", lambda: m.stat())
+    print(f"  {count3} messages (should be back to {count})")
+
+    step("DELE 1 (again, will commit)", lambda: m.dele(1))
+
+step("QUIT", lambda: m.quit())
+print("ALL DONE")
+
+# Reconnect to confirm the DELE really committed at QUIT.
+m2 = poplib.POP3_SSL("127.0.0.1", 995, context=ctx)
+m2.user(USER)
+m2.pass_(PASS)
+count4, size4 = m2.stat()
+print(f"after reconnect: {count4} messages (should be {count - 1})")
+m2.quit()