Add UnitVector tensor refinement

[connortsui20]

Depends on #9418.

Rationale for this change

Normalized currently stores normalized Vector values with the same dtype as arbitrary vectors, so the unit-norm invariant is not represented in the type.

What changes are included in this PR?

Adds the UnitVector extension dtype for vectors whose rows have norm 1.0 within tolerance or are exactly zero. Adds the L2Normalize scalar function and uses it in the Normalized encoding, where vector directions become UnitVector while fixed-shape tensor directions retain their dtype.

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

Adds the public UnitVector dtype and L2Normalize scalar function. InnerProduct and CosineSimilarity accept compatible Vector and UnitVector pairs.

[codspeed-hq]

Merging this PR will regress 2 benchmarks

⚠️ 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.

⚡ 6 improved benchmarks
❌ 2 regressed benchmarks
✅ 1992 untouched benchmarks
⏩ 89 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	words_gather_scalar[65536]	8.3 µs	9.4 µs	-12.02%
❌	Simulation	take[small_m/shuffled/primitive/nonnull/chunks=16384/indices=16]	1.1 ms	1.2 ms	-11.37%
⚡	Simulation	encode_nullable[256]	333.5 µs	231.5 µs	+44.08%
⚡	Simulation	encode_nullable[2]	1,109.2 µs	770.4 µs	+43.98%
⚡	Simulation	encode_nullable[32]	368.5 µs	259.4 µs	+42.08%
⚡	Simulation	encode_non_nullable[2]	866.5 µs	686.6 µs	+26.21%
⚡	Simulation	encode_non_nullable[256]	256.5 µs	209.3 µs	+22.56%
⚡	Simulation	encode_non_nullable[32]	304.8 µs	252.9 µs	+20.51%

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 ct/unit-vector (7a41686) with ct/extension-coerce-to (ec1dddb)}

Footnotes

1. 89 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. ↩