logtail: the deferred counter remembers a place in the log, not a second (stage 2b)

The counter passed over, while catching up after a start, every warning no
later than the last one counted - by time; mail.log stamps whole seconds,
so a second warning of that second, later in the file, was lost.

- The tailer gives each line its place (Place: the file's fingerprint and
  the offset just past the line): follow tracks the bytes the reader hands
  over, readRotated likewise in mail.log.1. Run hands tr.line to the
  counter.
- store.AddSenderCheckDeferred(at, n, place) counts, moves the
  last-deferral mark (never back - an upsert WHERE) and saves the place
  (settings sender_check_counted_place) in one transaction.
- Catching up, a warning in the file of the last one counted is passed over
  by place (end <= counted end); in another file - a rotated log read again
  - by time, as before. The fingerprint carries the inode, so the place
  stays true across a rotation.

Tests: two warnings of one second, an unclean stop between them, all lines
read again - each counted once; the store saves the place with the count
and keeps the latest deferral time. Existing counter tests pass a zero
place (time rule).

CHANGELOG [Unreleased], audit plan line ticked, roadmap 24/62.
Evidence: go vet ./... clean; go test ./internal/store, ./internal/logtail ok
(but for the Windows-only test); the server run follows before the push.

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

CHANGELOG.md

@@ -16,6 +16,12 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   in the log, with the time from which nothing was read. The log's
   fingerprint now includes the file's inode, so two files that begin alike
   are still told apart (audit 2026-10-12, stage 2b).
+- **A second deferral of the same second is counted after a restart.** The
+  Sender check's count of deferred mail remembered the time of the last
+  line it counted, to pass over lines read again after an unclean stop;
+  `mail.log` stamps whole seconds, so a second warning of that second was
+  passed over too. It now remembers the place in the log, saved with the
+  count in one transaction (audit 2026-10-12, stage 2b).
 
 ## [2.5.0] - 2026-10-14