Swap execute_range_query_pipeline's per-group bucket_map from HashMap to sorted Vec

[milindsrivastava1997]

execute_range_query_pipeline's step-major loop (#581 stage E.2) holds every group's bucket_map simultaneously instead of one at a time. Underlying accumulator data (all_data) was already fully resident before either loop shape — this is just per-group index overhead, but it's now paid for all groups at once.

fn build_bucket_map(
    buckets: &[crate::stores::TimestampedBucket],
) -> HashMap<u64, Vec<&dyn AggregateCore>> {
    let mut bucket_map: HashMap<u64, Vec<&dyn AggregateCore>> = HashMap::new();
    for ((start, _), bucket) in buckets {
        bucket_map.entry(*start).or_default().push(bucket.as_ref());
    }
    bucket_map
}

Fix: sorted Vec<(u64, &dyn AggregateCore)> + binary search instead of HashMap. No hash-table overhead, more compact per group — the win compounds now that N are held at once. Trade O(1) hash lookup for O(log n) binary search in window_buckets_for_step/single_window.

Not urgent — no signal yet that real group cardinality makes this matter. Filed to track, not to block.