feat(vortex-geo): add collect scalar function

[HarukiMoriarty]

Summary

• add strict homogeneous ST_Collect overloads for lists of Point, LineString, and Polygon
• return the corresponding native MultiPoint, MultiLineString, or MultiPolygon type
• use shared unary dispatch for constant handling and strict outer-null propagation
• reuse geometry storage on the all-valid path and compact only nullable inner elements
• add focused tests and a dedicated microbenchmark

[codspeed-hq]

Merging this PR will not alter performance

✅ 96 untouched benchmarks
⏩ 1858 skipped benchmarks¹

---

_{Comparing nemo/geo-collect (9428981) with nemo/geo-area (c6fd58a)²}

Footnotes

1. 1858 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. ↩

2. No successful run was found on nemo/geo-area (40f4666) during the generation of this report, so 7fc3956 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩