bench(h2o): window Top-N sweep over a declared-sorted input

[SubhamSinghal]

Which issue does this PR close?

Related to #6899 — adds benchmark coverage for the WindowTopN operators.

Rationale for this change

The existing h2o --subgroup window Top-N sweep (q13–q29) registers x with no declared ordering, so output_ordering() is None. Any plan that depends on the input being sorted is unreachable from those queries, however the data happens to be laid out on disk. This adds a window_sorted subgroup that publishes the same sweep over a WITH ORDER table, so ordering-dependent plans can be measured.

What changes are included in this PR?

A window_sorted h2o subgroup: 16 queries covering ROW_NUMBER / RANK / DENSE_RANK × 100 / 1K / 10K / 100K partitions, plus heavy-ties variants.

• Two load directives: the existing load_window_${SIZE}_${FORMAT}.sql creates x, then a new script writes a sorted copy via COPY (... ORDER BY pk, ob DESC) and registers it WITH ORDER (pk ASC, ob DESC). Reusing the existing loader keeps both the --size and --format axes working with no duplication. load is untimed, so the sort stays out of the measurement.

• WITH ORDER can only name columns, so the partition key (id3 % N) and the tie expression are materialized as pk and ob.

• Asserts guard the three ways this could silently measure nothing: both config flags took effect, the sorted copy holds every source row, and ob has the cardinality the query name claims.

• expect_plan PartitionedTopKExec is deliberately the shared substring, so the same file validates the heap operator and any streaming variant that replaces it.

  No Rust changes; benchmark files only.

  Are these changes tested?

  benchmark_runner h2o --subgroup window_sorted runs all 16 queries green. Row counts are exact: 2 per partition for the distinct-ORDER-BY shapes, and 1,010,812 (RANK) / 2,020,722 (DENSE_RANK) for the tie shapes — the top-1 and top-2 of 10 distinct values over 10M rows.

  Are there any user-facing changes?

  No.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.42%. Comparing base (408fc6f) to head (886ec22).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24732      +/-   ##
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- Coverage   81.42%   81.42%   -0.01%     
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  Files        1120     1120              
  Lines      402021   402021              
  Branches   402021   402021              
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- Hits       327357   327351       -6     
- Misses      55484    55488       +4     
- Partials    19180    19182       +2     

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