feat: record VERP envelope senders in forensic report archive copies

reports/opendmarc-reports.8.in

@@ -35,7 +35,14 @@ one file per report, named
 .IR seconds-pid-seq-label.eml .
 Each file contains the full message exactly as handed to SMTP (headers and
 MIME body), not just the gzipped XML or AFRF attachment, for later review
-of RFC compliance or message structure.
+of RFC compliance or message structure, with one exception: in forensic
+mode, the archived copy is prefixed with one or more non-standard
+.B #
+comment lines recording the envelope sender(s) actually used to send it,
+since that information (particularly the per-recipient address used when
+.I --verp
+is enabled) is otherwise never recorded anywhere -- the SMTP envelope
+leaves no trace in the message itself.
 .I label
 is the destination domain for aggregate reports, or the (first) recipient
 for forensic reports, with characters outside

reports/opendmarc-reports.in

@@ -762,7 +762,28 @@ if ($forensic)
 
 	my $archive_label = $rcpts[0];
 	$archive_label .= "+" . (scalar(@rcpts) - 1) if @rcpts > 1;
-	archive_report($archive_label, $message);
+
+	# Record the envelope sender(s) actually used for this report as a
+	# non-standard comment preamble on the archived copy.  VERP only
+	# changes the SMTP envelope (MAIL FROM), which leaves no trace in the
+	# message itself, so without this the archive can't show whether, or
+	# how, VERP fired.  Deliberately not formatted as RFC822 headers:
+	# archive_report()'s whole point is to capture the message exactly as
+	# it's handed to SMTP, and inventing header lines that were never
+	# really part of it would misrepresent that more than a few
+	# non-standard "#" lines on top do.
+	my $archive_preamble;
+	if ($verp)
+	{
+		$archive_preamble = "# VERP envelope senders used for this report:\n";
+		$archive_preamble .= "#   $_ -> " . verp_sender($repemail, $_) . "\n"
+			for @rcpts;
+	}
+	else
+	{
+		$archive_preamble = "# Envelope sender used for this report: $repemail\n";
+	}
+	archive_report($archive_label, $archive_preamble . $message);
 
 	$smtpfail = 0;