Function observe 

pub fn observe(
    state: PursuitState,
    ready: &[NodeId],
    in_flight: u32,
) -> PursuitVerdict

The stop condition, as a pure function of what the plan says and what is running.

Takes the state rather than the pursuit: the verdict does not depend on what the goal says, and a reader that had to hold a Pursuit to ask this question would need the depth-zero position to read the city. Declaring is the guarded act; looking is not.

No clock, no disk, no counter of its own: the caller already knows both facts, and a stop condition that went and looked for them could not be replayed.