Define RowFn and RowVisitor

[connortsui20]

Rationale for this change

• Tracking issue: Tracking Issue: Define the RowFn API #9129
• Epic: Epic: Row-oriented scalar functions #9128
• Execution tracking: Tracking Issue: Execute RowFn over Vortex arrays #9130

Defines the unstable author-facing RowFn contracts. The execution backend is split into #9353.

What changes are included in this PR?

Adds the unstable_row_fns feature boundary, RowFn and visitor traits, typed input and output contracts, tuple adapters, planning, and the blanket ScalarFnVTable scaffold. execute_rows remains a temporary error until #9353 adds the backend.

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

Adds vortex_array::scalar_fn::unstable::row behind the non-default unstable_row_fns feature. The API has no compatibility guarantees.

[codspeed-hq]

Merging this PR will regress 1 benchmark

⚠️ 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.

⚡ 5 improved benchmarks
❌ 1 regressed benchmark
✅ 1988 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	-12.04%
⚡	Simulation	take[small_m/shuffled/primitive/nonnull/chunks=1024/indices=256]	2.2 ms	1.9 ms	+14.99%
⚡	Simulation	take[small_m/shuffled/primitive/nonnull/chunks=1024/indices=1000]	5.7 ms	5 ms	+14.5%
⚡	Simulation	take[small_m/shuffled/primitive/nonnull/chunks=16384/indices=1000]	9.6 ms	8.5 ms	+13.85%
⚡	Simulation	take[small_m/shuffled/primitive/nonnull/chunks=16384/indices=256]	3.2 ms	2.9 ms	+10.49%
⚡	WallTime	words_gather_dispatch[1024]	33 ns	30 ns	+10%

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/row-fn-types (06a86d9) with develop (1a493b6)}

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. ↩