Function shards_owed 

pub fn shards_owed(
    root: &Path,
    group: &'static BenchmarkGroup,
    sha: &str,
    wanted: usize,
) -> Vec<(usize, usize)>

2026-09-26: The (index, count) shards of group a certification at sha still owes, for a planner that wants wanted shards:

• nothing, when the clean standing shards already form a complete partition;
• else the indices missing from the most complete partition measured at sha, whatever its count (a partition is never assembled across commits);
• else 0..wanted.

A shard that fails shard_problems is owed again. A group with no readable baseline or coverage entry owes 0..wanted.