perf: Descending vector cursors.

[gianm]

This patch adds descending-order vector cursors, enabling faster execution for timeseries, timeBoundary, and scan queries that run in descending order.

Benchmark                                      (descending)  (numSegments)  (queryGranularity)  (rowsPerSegment)         (schemaAndQuery)  (vectorize)  Mode  Cnt      Score     Error  Units
TimeseriesBenchmark.querySingleQueryableIndex          true              1                hour            750000  basic.timeFilterNumeric        false  avgt   15  11877.122 ±  89.831  us/op
TimeseriesBenchmark.querySingleQueryableIndex          true              1                hour            750000  basic.timeFilterNumeric         true  avgt   15   2620.778 ±  18.603  us/op
TimeseriesBenchmark.querySingleQueryableIndex         false              1                hour            750000  basic.timeFilterNumeric        false  avgt   15  12219.221 ± 106.745  us/op
TimeseriesBenchmark.querySingleQueryableIndex         false              1                hour            750000  basic.timeFilterNumeric         true  avgt   15   2468.444 ±  57.393  us/op

[FrankChen021]

Severity	Findings
P0	0
P1	0
P2	1
P3	0
Total	1

Reviewed 26 of 26 changed files.

Validation: git diff --check passed; no builds or tests were run.

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This is an automated review by Codex GPT-5.6-Luna(max)

[FrankChen021]

[P2] Reverse factory bypasses virtual-column group-by handling

The wrapper inherits the generic makeGroupByVectorColumnSelector implementation instead of preserving the underlying factory's virtual-column specialization. A descending vectorized group-by using an ExpressionVirtualColumn can therefore reach the generic ARRAY processor and fail with UnsupportedOperationException instead of using the supported deferred expression selector. Add a reverse-aware group-by override and regression coverage.

[gianm]

makeGroupByVectorColumnSelector won't be called anyway since groupBy doesn't use descending cursors. I'll add an explicit override that throws NotYetImplemented.

[FrankChen021]

Severity	Findings
P0	0
P1	0
P2	1
P3	0
Total	1

Reviewed 26 of 26 changed files.

---

This is an automated review by Codex GPT-5.6-Luna(max)

[FrankChen021]

[P2] Preserve the reverse-iterator binary signature

Changing the public method return type from IntIterator to PeekableIntIterator changes its JVM descriptor. Precompiled callers using the previous processing artifact can fail with NoSuchMethodError at runtime. Preserve the IntIterator-returning method and expose the peekable iterator under a new method name.

[gianm]

I am not worried about this, this function is an internal API.

[FrankChen021]

Reviewed 26 of 26 changed files. The follow-up concern is handled: normal groupBy cursor specs do not request descending ordering, and the explicit NotYetImplemented guard addresses the original concern. No further inline reply is needed.

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This is an automated review by Codex GPT-5.6-Luna(max)