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>

docs/audit-2026-10-12.md

@@ -751,7 +751,7 @@ order - each stands on the one before.
    rotated twice: logged, with from when to when nothing was read. A test
    that stops the tailer, appends, rotates, appends, starts it, and finds
    both halves once. - **Opus**
-- [ ] **The deferred counter remembers a place in the log, not a second**
+- [x] **The deferred counter remembers a place in the log, not a second**
    (owner, 2026-10-14, from the Not done list of from-header-binding; he
    chose it over leaving the case written down). After an unclean stop the
    tailer re-reads lines it had read, and the counter passes over every