Message.received_at, CalendarEvent.start_time/end_time,
CalendarSyncRequest/ListEventsRequest.start_window/end_window,
FilterRule.created_at/updated_at, FilterExecutionLog.matched_at,
AccountConfig.sync_interval_secs, PreviewRuleResponse.execution_time_us,
and AuditRecord.timestamp_ns all mixed unix-seconds and nanosecond int64
scalars, leaving the unit ambiguous at the wire level. Move every wall-clock
field to google.protobuf.Timestamp and every duration to
google.protobuf.Duration, add prost-types as the standard well-known-types
crate, and add nuncio_proto::time as the single conversion point between
the store's native integer instants and the wire types (including a
lossless unix-nanos split/join for the WORM audit ledger's sub-second
precision). Update the daemon's gRPC mappers and the CLI's request
construction/rendering accordingly, and regenerate the descriptor golden.
Refs #274
James Maes