[Variant] Normalize marked Variant arrays at the native Parquet boundary

[peterxcli]

Problem

Once Variant identity exists in the native schema, ordinary Parquet can still return metadata, residual value, and optional typed_value in a physical Struct that is not Spark's required whole-value layout.

Parent: #5546
Depends on: #5548 (merged in #5631)
Implementation: #5715 (in review)
Related: #3983

Solution

• Route only explicitly marked Variant Fields to a dedicated schema-adapter normalization path.
• Construct Arrow-rs VariantArray by child name.
• Call unshred_variant once at the native Parquet boundary.
• Remove typed_value and rebuild exactly [value, metadata] as ordinary Binary children.
• Preserve parent nulls and the original marked output Field.
• Reuse feat: support unicode case sensitive field names for reading parquet #5602's shared Parquet schema-name matching (Support Spark-compatible Unicode case-insensitive Parquet field matching #5495 is complete); verify that Unicode name remapping retains the Variant normalization wrapper.
• Handle the canonical and legacy Spark object-key ordering required by supported Spark profiles.

The JVM scan gate remains closed in this task. It must not add subfield pruning, predicate pushdown, shredded writing, or JVM-side unshredding.

Tests

• Canonical unshredded, fully shredded, and partially shredded values agree.
• Inputs whose children are not in Spark output order are reordered.
• Objects, arrays, scalars, JSON null, SQL NULL, nested residuals, and nullable parents round-trip.
• typed_value never reaches the output.
• The output Field marker and parent null bitmap are preserved.
• Remove Variant UTF-16 output rewriting #5474 is linked to removable legacy ordering compatibility code.

[peterxcli]

take