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/imaptest.py

@@ -0,0 +1,68 @@
+import imaplib
+import ssl
+import sys
+
+HOST = "127.0.0.1"
+USER = "barant@barant.com"
+PASS = "imaptest123"
+
+ctx = ssl.create_default_context()
+ctx.check_hostname = False
+ctx.verify_mode = ssl.CERT_NONE
+
+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 = imaplib.IMAP4_SSL(HOST, 993, ssl_context=ctx)
+print("connected:", m.welcome)
+
+step("LOGIN", lambda: m.login(USER, PASS))
+step("CAPABILITY", lambda: m.capability())
+step("LIST", lambda: m.list())
+typ, data = step("SELECT INBOX", lambda: m.select("INBOX"))
+print("  EXISTS:", data)
+
+typ, data = step("SEARCH ALL", lambda: m.search(None, "ALL"))
+ids = data[0].split()
+print(f"  {len(ids)} messages")
+
+if ids:
+    first = ids[0]
+    typ, data = step("FETCH ENVELOPE", lambda: m.fetch(first, "(ENVELOPE)"))
+    print("  ", data)
+    typ, data = step("FETCH FLAGS UID", lambda: m.fetch(first, "(FLAGS UID)"))
+    print("  ", data)
+    typ, data = step("FETCH BODY[]", lambda: m.fetch(first, "(BODY[])"))
+    print("  body length:", len(data[0][1]) if data and data[0] else 0)
+    step("STORE +FLAGS \\Flagged", lambda: m.store(first, "+FLAGS", "\\Flagged"))
+    typ, data = step("SEARCH FLAGGED", lambda: m.search(None, "FLAGGED"))
+    print("  flagged ids:", data)
+    step("STORE -FLAGS \\Flagged", lambda: m.store(first, "-FLAGS", "\\Flagged"))
+
+step("STATUS INBOX", lambda: m.status("INBOX", "(MESSAGES UIDNEXT UIDVALIDITY UNSEEN)"))
+
+# APPEND a literal message, then verify it shows up, then clean it up via COPY+EXPUNGE
+msg = b"From: test@barant.com\r\nTo: barant@barant.com\r\nSubject: IMAP APPEND test\r\n\r\nHello from APPEND.\r\n"
+typ, data = step("APPEND", lambda: m.append("INBOX", "", None, msg))
+print("  ", data)
+
+typ, data = step("SEARCH SUBJECT", lambda: m.search(None, 'SUBJECT "APPEND test"'))
+appended_ids = data[0].split()
+print("  found:", appended_ids)
+
+if appended_ids:
+    step("COPY to Trash", lambda: m.copy(appended_ids[-1], "Trash"))
+    step("STORE +FLAGS \\Deleted", lambda: m.store(appended_ids[-1], "+FLAGS", "\\Deleted"))
+    step("EXPUNGE", lambda: m.expunge())
+
+typ, data = step("SELECT Trash", lambda: m.select("Trash"))
+print("  Trash EXISTS:", data)
+
+step("LOGOUT", lambda: m.logout())
+print("ALL DONE")