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/stresstest.py

@@ -0,0 +1,75 @@
+import imaplib, poplib, smtplib, ssl, threading, time, random
+
+ctx = ssl.create_default_context()
+ctx.check_hostname = False
+ctx.verify_mode = ssl.CERT_NONE
+
+USER = "barant@barant.com"
+PASS = "imaptest123"
+
+errors = []
+lock = threading.Lock()
+
+def note_error(name, e):
+    with lock:
+        errors.append(f"{name}: {e}")
+
+def imap_cycle(i):
+    try:
+        m = imaplib.IMAP4_SSL("127.0.0.1", 993, ssl_context=ctx)
+        m.login(USER, PASS)
+        m.select("INBOX")
+        m.search(None, "ALL")
+        typ, data = m.fetch("1", "(FLAGS UID)")
+        m.logout()
+    except Exception as e:
+        note_error(f"imap#{i}", e)
+
+def pop3_cycle(i):
+    try:
+        m = poplib.POP3_SSL("127.0.0.1", 995, context=ctx)
+        m.user(USER)
+        m.pass_(PASS)
+        m.stat()
+        m.list()
+        m.quit()
+    except Exception as e:
+        note_error(f"pop3#{i}", e)
+
+def smtp_cycle(i):
+    try:
+        s = smtplib.SMTP("127.0.0.1", 25, timeout=10)
+        s.ehlo("stress-client")
+        msg = f"From: stress{i}@barant.com\r\nTo: barant@barant.com\r\nSubject: stress {i}\r\n\r\nstress body {i}\r\n"
+        s.sendmail(f"stress{i}@barant.com", ["barant@barant.com"], msg)
+        s.quit()
+    except Exception as e:
+        note_error(f"smtp#{i}", e)
+
+def rude_disconnect(i):
+    # Connect and drop mid-handshake / mid-command without clean close -
+    # exercises the abrupt-disconnect path specifically.
+    import socket
+    try:
+        s = socket.create_connection(("127.0.0.1", 993), timeout=5)
+        s.recv(50)
+        s.close()
+    except Exception as e:
+        note_error(f"rude#{i}", e)
+
+threads = []
+N = 40
+for i in range(N):
+    fn = random.choice([imap_cycle, pop3_cycle, smtp_cycle, rude_disconnect])
+    t = threading.Thread(target=fn, args=(i,))
+    threads.append(t)
+    t.start()
+    time.sleep(0.02)
+
+for t in threads:
+    t.join(timeout=15)
+
+print(f"Completed {N} concurrent mixed-protocol cycles.")
+print(f"Errors: {len(errors)}")
+for e in errors[:20]:
+    print(" ", e)