from-header-binding: what the reviews had left as found, and address extensions

The owner on the three things the reviews' commit left as found: can they be
fixed? They can.

- Downtime that crossed a UTC midnight was all booked on the day of the
  restart: it is cut at the midnights between (byDay).
- A message with the null envelope sender - a bounce an application writes
  itself - left a refusal row without a domain, which no domain
  administrator would see: the row takes the domain the login is bound to.
- A log read again after an unclean stop counted its deferrals again: while
  the tailer catches up, a warning no later than the last one counted is
  passed over. The mark is kept to the fraction of a second the log gives.

And on the check being stricter than Postfix for an address with an
extension (invoices+x@ passed in the envelope, was refused in the header):
it makes Postfix's second try now, the address without its extension.
recipient_delimiter was the distribution's default; it is set in
postfix-config.sh, since the check depends on it. e2e sends one through.

Test server, this tree: go vet ok, go test 23 packages ok, make e2e ok (339 s).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/guide.md

@@ -1097,6 +1097,10 @@ listed for it. Since `2.4.0` that rule covers both senders a message has:
 - the **`From:` line** of the message - the one a mail program shows and the
   one the DKIM signature is chosen by - which the panel checks.
 
+An address with an extension counts as the address: an application listed as
+`invoices@…` may also send as `invoices+march@…`, in the envelope and in
+`From:` alike.
+
 A message whose `From:` is anything else is refused with `550 5.7.1` and a
 text naming the line: another of your domains, the server's own name, an
 address at a domain that is not on this server, a line with two addresses,