add method
Records entry unless it is blank or a consecutive duplicate of the most
recent entry, capping to maxEntries. Returns whether the buffer changed,
so callers can skip persisting on a no-op.
Implementation
bool add(String entry) {
if (entry.trim().isEmpty) return false;
if (_entries.isNotEmpty && _entries.last == entry) return false;
_entries.add(entry);
_cap();
return true;
}