Use u8 Union type IDs

[connortsui20]

Tracking issue: #8769 and #7882

Splits the type-ID foundation for #8769 into its own change.

• changes Union variant type IDs from i8 to u8 across the dtype APIs, protobuf, FlatBuffers, and generated bindings
• supports the full u8 tag range
• supports up to 256 Union variants with consecutive tags 0..=255

[github-actions]

Polar Signals Profiling Results

Latest Run

Status	Commit	Job	Attempt	Link
🟢 Done	2eb8864		1	Explore Profiling Data

Previous Runs (4)

Status	Commit	Job	Attempt	Link
🟢 Done	378b981		1	Explore Profiling Data
🟢 Done	2c72940		1	Explore Profiling Data
🟢 Done	d9583d3		1	Explore Profiling Data
🟢 Done	7820b66		1	Explore Profiling Data

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[github-actions]

Benchmarks: Vortex queries

Verdict: No clear signal (low confidence)
Attributed Vortex impact: +1.9%
Engines: DataFusion No clear signal (+9.6%, medium confidence) · DuckDB No clear signal (-5.4%, low confidence)
Vortex (geomean): 0.947x ➖
Parquet (geomean): 0.938x ➖
Shifts: Parquet (control) -6.2% · Median polish -5.1%

How to read Verdict and Engines

• Verdict: Overall PR-level signal after subtracting baseline drift estimated from Parquet control rows. It can be Likely improvement, Likely regression, or No clear signal.
• Engines: Per-engine attribution. DataFusion is compared against DataFusion/Parquet controls; DuckDB is compared against DuckDB/Parquet controls. This answers whether each engine improved or regressed independently.
• Confidence: Based on directional consistency, share of rows above the noise floor, and control-run noise.

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datafusion / vortex-file-compressed (0.990x ➖, 0↑ 0↓)

name	PR 2eb8864 (ns)	base 8f75dbe (ns)	ratio (PR/base)
vortex_q00/datafusion:vortex-file-compressed	9935024	9766676	1.02
vortex_q01/datafusion:vortex-file-compressed	6252967	6494068	0.96

datafusion / parquet (0.903x ➖, 1↑ 0↓)

name	PR 2eb8864 (ns)	base 8f75dbe (ns)	ratio (PR/base)
vortex_q00/datafusion:parquet 🚀	19415579	21678477	0.90
vortex_q01/datafusion:parquet	4472598	4916579	0.91

duckdb / vortex-file-compressed (0.923x ➖, 0↑ 0↓)

name	PR 2eb8864 (ns)	base 8f75dbe (ns)	ratio (PR/base)
vortex_q00/duckdb:vortex-file-compressed	9914440	10671677	0.93
vortex_q01/duckdb:vortex-file-compressed	6042909	6589104	0.92

duckdb / parquet (0.975x ➖, 0↑ 0↓)

name	PR 2eb8864 (ns)	base 8f75dbe (ns)	ratio (PR/base)
vortex_q00/duckdb:parquet	23279550	23744732	0.98
vortex_q01/duckdb:parquet	9377013	9666241	0.97

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No file size changes detected.