feat(vortex-spatial): add length scalar function

[HarukiMoriarty]

Rationale

Vortex native LineString arrays should support planar length without an Arrow, WKB, or row-oriented geo_types conversion.

What changes are included?

• Add vortex.st.length for native LineString inputs.
• Compute every row directly from flattened coordinate buffers and row offsets.
• Sum all adjacent segments, supporting empty, one-point, and arbitrary-length LineStrings.
• Preserve constant results and strict null propagation.
• Add focused correctness tests and CodSpeed benchmarks.

Split

This was split from #9201. It is standalone on develop; #9201 now contains only ST_MakeLine and primitive Interleave support.

[codspeed-hq]

Merging this PR will degrade performance by 13.85%

❌ 2 regressed benchmarks
✅ 1935 untouched benchmarks
🆕 3 new benchmarks
⏩ 85 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	cold_misaligned[(64, 256)]	4.4 ms	5.3 ms	-17.24%
❌	Simulation	take[small_m/shuffled/primitive/nonnull/chunks=16384/indices=16]	1.2 ms	1.3 ms	-10.33%
🆕	Simulation	nullable_two_vertex_lines	N/A	384.6 µs	N/A
🆕	Simulation	sixteen_vertex_lines	N/A	805.5 µs	N/A
🆕	Simulation	two_vertex_lines	N/A	335.2 µs	N/A

Tip

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

---

_{Comparing nemo/geo-length (3fec089) with develop (19f771f)}

Footnotes

1. 85 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. ↩