Add DType::Union variant carrying just Nullability

[connortsui20]

Summary

Tracking issue: #7882

Adds the Union(Nullability) enum variant, plumbs it through all existing match arms with todo!("TODO(connor)[Union]: unimplemented"), and reserves slot 12 in the flatbuffer and protobuf wire schemas.

A follow-up PR will replace Union(Nullability) with Union(UnionVariants, Nullability) and fill in the serde paths.

Note that this PR doesn't have any implementation logic, it is just boilerplate.

API Changes

Adds a new DType::Union variant.

Testing

Nothing to test.

[codspeed-hq]

Merging this PR will degrade performance by 21.7%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 2 regressed benchmarks
✅ 1208 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	varbinview_zip_block_mask	2.9 ms	3.7 ms	-21.7%
❌	Simulation	varbinview_zip_fragmented_mask	6.2 ms	6.9 ms	-11.29%

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_{Comparing ct/add-union-boilerplate (06f2495) with develop (c663fa9)}