DeltaSetAggregator per-step keys replay is O(range_length^2), unbounded by lookback #599

`execute_range_query_pipeline`'s per-step keys merge (#583) recomputes each output step's key set from scratch, scanning `[0, current_time)` for `DeltaSetAggregator` (since its window is unbounded, replaying from the beginning by design — see `docs/583-range-keys-per-step-design.md`, Q2/Q3). For `N` output steps, total keys-merge work is `O(N^2)` bucket-merges (step 1 processes ~1 bucket, step 2 processes ~2, ..., step N processes ~N).

This was a deliberate, documented tradeoff in #583's design discussion: ship the simple recompute-from-scratch version first (easiest to verify correct, matches the existing values-loop pattern exactly), and only build incremental carry-forward merging (an `O(N)` alternative — persist a running merged accumulator across the `current_time` loop, folding in only new buckets since the last step) if this is ever a measured problem in practice. `SetAggregator`'s window is bounded by its own `window_size_ms` regardless of range length, so it never has this blowup — only `DeltaSetAggregator` does.

Filing this so the tradeoff is trackable outside the design doc/PR discussion, not because it's known to be a current problem. No action needed unless/until it's measured to matter for some real range/step combination.