Add Map logical type and scalar support

[AdamGS]

Rationale for this change

Vortex currently has no native logical map type, so Arrow maps must be represented as lists of structs and lose map-specific semantics such as non-null keys and the keys_sorted assertion. This provides the type-level foundation for native Map support; #9108 builds on it with canonical arrays and builders. Related to #428.

What changes are included in this PR?

• Adds DType::Map and MapDType, including key/value dtypes, outer nullability, sortedness, validation, coercion, display, equality, and hashing.
• Adds map scalar construction and typed access through Scalar::map, Scalar::try_map, and MapScalar.
• Adds map dtype and scalar serialization through Serde, Protobuf, and FlatBuffers.
• Adds Arrow dtype/schema round-tripping, Vortex-to-Arrow map scalar conversion, and DataFusion physical-schema reconciliation.
• Tests cover dtype and scalar validation, null and empty maps, coercion, serialization round-trips, Arrow schema conversion, and nested extension metadata.

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

This adds the public DType::Map and ScalarValue::Map variants, MapDType, MapScalar, and their constructors and accessors. Consumers that exhaustively match DType or ScalarValue must handle the new variants. The Protobuf and FlatBuffers dtype schemas also gain Map variants.

This PR supports Map dtypes, schemas, and scalars, but not full Arrow MapArray transport.

[codspeed-hq]

Merging this PR will regress 1 benchmark

⚡ 3 improved benchmarks
❌ 1 regressed benchmark
✅ 1838 untouched benchmarks
⏩ 44 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	compact_sliced[(4096, 90)]	780.3 ns	867.8 ns	-10.08%
⚡	Simulation	new_raw_prim_test_between[i32, 2048]	39.1 µs	33.9 µs	+15.35%
⚡	Simulation	execute_scalar_struct_simple	483.2 µs	427.2 µs	+13.12%
⚡	Simulation	execute_scalar_struct_wide	2.3 ms	2 ms	+11.36%

Tip

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

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_{Comparing adamg/maptype-1 (6889de1) with develop (02f89d6)}

Footnotes

1. 44 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. ↩