test(promql): cover nested temporal anchors and topk ordering #771

Goal

Extend PromQL differential coverage for two semantics that the current 90-query nested-aggregation suite intentionally normalizes away or does not exercise.

Scope

1. Add an opt-in order-sensitive comparison mode for instant-vector responses in promql-compliance/runner.

   • Keep the default comparison order-insensitive for general vectors and matrices.
   • Use the mode for an instant topk case so ASAPQuery is compared against Prometheus in returned descending-value order.
   • Assert full-label-set ordering for equal values.
   • Do not require one global ordering for /query_range matrices.

2. Add nested aggregation cases with temporal inner anchors, each exercised through both /query and /query_range:

   • e.g. topk(3, sum by (job) (rate(data[5m])))
   • at least one reducer after a temporal anchor, such as avg(sum by (job) (rate(data[5m]))).

Rationale

The existing 90-query suite uses bare data selectors and normalizes result samples by labels before comparison. It verifies result membership and values, but cannot detect TopK presentation-order regressions and does not cover a range selector consumed by a temporal function before the nested aggregation pipeline.

Acceptance criteria

• A direct Prometheus-vs-ASAPQuery instant TopK order check passes.
• The existing default differential comparisons remain order-insensitive.
• Nested temporal-anchor queries pass against Prometheus for both instant and range endpoints.
• The suite continues to verify range-at-t / instant-at-t parity.