#4417 Docs: usage-telemetry reference should describe the telemetry stats empty state and the last-flush record

swamp-club#3833 changes swamp telemetry stats so its empty state explains why it is empty. The CLI keeps a small last-flush record (flush-state.json beside the user-global spool: last attempt time and result, last success time and entry count). When stats finds no entries it still prints No telemetry data found, then one line: telemetry is opted out (user-level preference or SWAMP_NO_TELEMETRY / DO_NOT_TRACK), or the spool is empty because the last flush sent N entries at a given time (with a pointer to the telemetryKeepFlushed repository key), or the last attempt failed, or nothing has been recorded yet. JSON output keeps message and adds telemetryEnabled, disabledBy, lastFlush and keepFlushedKey fields.

Manual page to update: content/manual/reference usage-telemetry-events, Local spool section. Add flush-state.json to the spool description, describe the empty-state output in log and JSON mode, and state that with telemetryKeepFlushed on, stats counts the kept .flushed.json entries. Land after #3833 ships.