feat(vortex-spatial): preserve GeoArrow metadata

[HarukiMoriarty]

Rationale for this change

Vortex preserved GeoArrow crs metadata but dropped crs_type and edges, making Arrow round-trips lossy.

What changes are included in this PR?

• Add crs_type and edges to SpatialMetadata with backward-compatible protobuf encoding.
• Preserve all three fields through spatial Arrow import/export and WKB conversion.
• Move shared metadata and geometry helpers out of extension/mod.rs.
• Add metadata serialization and GeoArrow round-trip tests.

What APIs are changed? Are there any user-facing changes?

Exports CrsType and Edges. Spatial computation is unchanged; geo kernels remain planar.

[codspeed-hq]

Merging this PR will not alter performance

✅ 1962 untouched benchmarks
⏩ 89 skipped benchmarks¹

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_{Comparing nemo/geo-metadata (1ec22c0) with develop (ca7f626)}

Footnotes

1. 89 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. ↩