Execute owned RowFn outputs over valid rows

[connortsui20]

Rationale for this change

• Stacked on: Implement RowFn batch execution #9450
• Tracking issue: Tracking Issue: Execute RowFn over Vortex arrays #9130
• Epic: Epic: Row-oriented scalar functions #9128

Adds direct valid-row execution for owned RowFn outputs before the first nullable deferred owned consumer.

What changes are included in this PR?

Adds Default to OutputElement and uses it only to initialize skipped output positions. Batch execution masks those positions before returning the array.

Owned infallible and deferred visits execute directly over null-tolerant decoded inputs. Unsupported encodings continue to decline this path; #9349 adds the later filter-and-scatter fallback.

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

Extends the unstable OutputElement contract with Default. There are no stable API changes.

[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.

⚡ 2 improved benchmarks
❌ 1 regressed benchmark
✅ 1953 untouched benchmarks
⏩ 442 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_varbinview_opt_into_canonical[(10, 100)]	592.2 µs	660.3 µs	-10.31%
⚡	Simulation	take[duplicates/repeated/primitive/nonnull/chunks=16/indices=1000]	270.5 µs	228.9 µs	+18.19%
⚡	Simulation	compress_fsst[(500, 64, 4)]	517.2 µs	467.6 µs	+10.61%

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-owned-valid-rows (d56d815) with develop (ae156bf)²}

Footnotes

1. 442 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. ↩

2. No successful run was found on develop (cb77d9a) during the generation of this report, so ae156bf was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩