v3 spec types — Variant, Geometry, Geography, Unknown

### Is your feature request related to a problem or challenge?

Forward-compat with v3 tables produced by Spark. We must extend PrimitiveType enum + Arrow type mapping + Avro schema + serde. Reference Java org.apache.iceberg.types.Types.

### Describe the solution you'd like

_No response_

### Willingness to contribute

I can contribute to this feature independently

Start with the PrimitiveType enum and trace its Arrow type mapping, Avro schema handling, and serde support. Compare each area with the referenced Java org.apache.iceberg.types.Types implementation; the work is done when Variant, Geometry, Geography, and Unknown support v3 tables produced by Spark across those paths.

Written by the indexing model from the issue text.