inbound journal: one row per recipient, and every row counts to the cap (audit M3)

Security audit 2026-10-17, finding M3, stage 1. inboundSession.RcptTo
appended every RCPT (the outbound session de-duplicates), and the end of
the message inserted a row per recipient one statement at a time inside
Postfix's callback; the cut to 10 000 rows every 500 counted only the log
tailer's rows. Postfix accepts a repeated recipient up to
smtpd_recipient_limit (1 000) and sends each to the milter, so one message
was a thousand rows, uncut until the six-hourly prune.

Now: the inbound milter folds a repeated recipient (case-insensitive, as
cleanup's been_here() with BH_FLAG_FOLD does, and as session.RcptTo does);
a message's rows go in one transaction (store.InsertInboundSpamMessage,
which replaces InsertInboundSpam in milter.InboundStore); the count and the
cut moved from the tailer into the store (countInboundWrites, an atomic
counter on Store) so the milter's rows, the tailer's and the tailer's
start-up re-reads all count. store.InboundSpamLogMaxRows is the one cap.

Tests: TestInboundRecipientRepeatedIsOneRow (1 000 RCPT in two cases, one
row); TestInboundJournalIsCutWhicheverWriterWrites (a small cap: a message's
rows and refusals together cross the mark, the table is cut); the tailer's
own counting test went with its counter. go test ./internal/store
./internal/milter ./internal/logtail (Windows-only Follow failures aside).
Also gofmt of a test line from the M2 commit.

Docs: security.md § Mail path, architecture.md; CHANGELOG; plan.

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

docs/security.md

@@ -463,7 +463,13 @@ operator can act on it.
   `2.14.4` the reply ended at the *first* `; from=<`, which a stranger's
   recipient could supply - security audit 2026-10-17, M2.)
   Only a recipient at an inbound domain makes a row. The journal is cut to
-  10 000 rows every 500 rows the tailer writes, and by age every six hours.
+  10 000 rows every 500 rows written - the tailer's refusals and the journal
+  milter's accepted recipients together, counted in the store - and by age
+  every six hours. The journal milter writes a recipient given twice, in any
+  case, once, and a message's rows in one transaction. (Until `2.14.4` it
+  wrote every repeated RCPT and its rows were not counted toward the cut:
+  one message could be a thousand rows, uncut for six hours - security
+  audit 2026-10-17, M3.)
 - **Filter lists** reach another container as files: values are whitelisted on
   the way in (address, domain, canonical IP or network; no `/0`, no
   IPv4-mapped network), the writer refuses a stored value with whitespace, a