Function collapse_below 

pub fn collapse_below(
    log: &dyn OpLog,
    acks: &AckVector,
    sink: &dyn SnapshotSink,
    snapshot_for_array: impl Fn(&str, Hlc) -> ArrayResult<Option<TileSnapshot>>,
) -> ArrayResult<GcReport>

Compact ops below the min-ack frontier into snapshots, then prune the log.

Algorithm:

1. Determine frontier = acks.min_ack_hlc(). If None, return early with an empty report — GC must not proceed without knowing every peer’s progress.
2. Collect distinct array names whose ops have hlc < frontier.
3. For each such array, call snapshot_for_array(name, frontier).
   • Some(snap) → write via sink.write_snapshot(&snap) and increment snapshots_written.
   • None → the caller signals “no live state for this array”; skip the snapshot write (ops will still be dropped).
   • Err → propagate immediately; the log is not mutated.
4. After all snapshots succeed, call log.drop_below(frontier) and record the count in ops_dropped.
5. Return the report.