Function evaluate 

pub fn evaluate(
    candidates: &[QueueCandidate],
    config: &QueueConfig,
) -> QueueDecision

Evaluate the queue and decide which torrents to start/stop.

Negative limits mean “unlimited” for that category. Checking torrents are processed first (transient, shouldn’t be blocked by DL/seed slot accounting). They count toward active_limit but NOT toward active_downloads or active_seeds. Seeding group is sorted by seed_rank descending (higher demand first), breaking ties by queue position.