Which attempt of a run is current - the fence §7.5 and §7.6 rely on.
A monotonic counter per run, starting at [AttemptGeneration::FIRST]. An event, an MCP call or a session lookup carrying an older generation may not touch status, control or authority; see [crate::AttemptGeneration::admits]. It is a counter rather than an id so "older" is a comparison, not a lookup.
