log tailer: the rotated mail.log read until postlogd lets it go

postlogd opens maillog_file once per process (Postfix 3.7.11,
src/postlogd/postlogd.c, pre_jail_init()) and leaves it only when it
exits: at "postfix reload" master closes the service's status pipe
(src/master/master_service.c, master_restart_service() ->
master_status_cleanup()) and dgram_server_abort() exits on reading its end
(src/master/dgram_server.c). logrotate renames and creates, then runs the
reload (build/logrotate-mail.conf postrotate; logrotate 3.21.0,
logrotate.c, rotateLogSet(), after rotateSingleLog()), so lines go to the
renamed file in between, and the tailer, already on the new one, never
read them (assumptions A-33). follow() now keeps the renamed file open as a
rotatedTail and reads it on each tick for rotatedGrace (a minute, far
longer than the gap); its places are never saved as the resume point, so a
restart still reads the rotated file through readRotated. A truncated file
is not kept.

The RFC 3339 stamp goes (A-12): it was written for a maillog_file_format
said to arrive in Postfix 3.9; there is none in 3.7.11 nor in the 3.12
snapshot of 2026-09-25 (proto/postconf.proto; msg_logger.c,
msg_logger_print() keeps strftime("%b %d %H:%M:%S ")). Every fixture now
carries the stamp Postfix writes; logtail's TestMain fixes time.Local to
UTC, since the stamp is local time and this PC is UTC+3.

Tests: TestFollowReadsTheRotatedFileAfterTheSwitch (Linux: a line appended
to the renamed file after the switch reaches the store). Stand
(2026-10-14): go vet and go test ./... pass on the server (23 packages);
make e2e exit 0, delivery_log_survives_rotation included. The tested tree
was this commit plus the previous one; the CHANGELOG entry was written
after the run, and a comment's logrotate citation corrected
(rotateLogSet runs postrotate, not rotateSingleLog).

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

internal/logtail/inbound.go

@@ -269,13 +269,6 @@ func printable(s string, max int) string {
 // process has the container's zone (TZ is in import_environment,
 // src/global/mail_params.h). A line with no readable stamp is dated now.
 func lineTime(line string, now time.Time) (time.Time, string) {
-	if m := isoStampRe.FindStringSubmatch(line); m != nil {
-		rest := strings.TrimSpace(line[len(m[0]):])
-		if t, err := time.Parse(time.RFC3339Nano, strings.TrimSpace(m[0])); err == nil {
-			return t.UTC(), rest
-		}
-		return now, rest
-	}
 	if m := syslogStampRe.FindStringSubmatch(line); m != nil {
 		rest := strings.TrimSpace(line[len(m[0]):])
 		stamp := strings.Join(strings.Fields(m[1]), " ")