A crash between the JSONL append and the meta update in addMsg can
leave meta.Count stale (e.g. file has 101 lines but meta says 100).
The previous code only reconciled when Count==0, so a nonzero stale
count was silently trusted, causing keepLast/skip to be calculated
against the wrong total.
Now TruncateHistory always counts the actual lines on disk. This is
cheap (scan without unmarshal) and TruncateHistory is not a hot path.
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