Fix sortedness for singleton sequences

[connortsui20]

Superseded by #10269, which groups this change with the related statistics work. The original commits and branch are preserved.

Original PR description

Tracking Issue: #10177

Summary

A singleton has no adjacent pair that can be out of order. Sequence currently seeds sortedness from its step alone, so a descending singleton is reported unsorted and a zero-step singleton is reported not strictly sorted. The cached answer takes precedence over the aggregate helper's length check.

Changes

Use the sequence length in both constructor hints and the encoding-specific aggregate kernel. Longer sequences keep the existing step rules. Keeping this correction separate lets the producer migration preserve a defined sortedness contract.

Stack

Depends on #10267 in migration stack #10229.

[codspeed]

Merging this PR will regress 1 benchmark

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 2101 untouched benchmarks
⏩ 503 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	compress_fsst[(200, 4, 8)]	127.8 µs	147.9 µs	-13.59%
⚡	Simulation	bench_compare_sliced_dict_primitive[(2500, 10000)]	82.4 µs	28.7 µs	×2.9

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing ct/stats-sequence-boundaries (f46a4e1) with ct/stats-nullability-proof (bb58423)}

Footnotes

1. 503 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩