vortex-io: Add explicit span on spawn_io

[brancz]

Summary

I/O tasks are likely to cause a lot of tracing to be produced. With an explicit span on them, we can control it more precisely.

API Changes

No API changes.

Testing

No testing, this just allows consumers to configure tracing at a more granular level.

@AdamGS

[codspeed-hq]

Merging this PR will improve performance by 20.94%

⚠️ 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.

⚡ 7 improved benchmarks
❌ 1 regressed benchmark
✅ 1267 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	bitwise_not_vortex_buffer_mut[128]	246.1 ns	275.3 ns	-10.6%
⚡	Simulation	chunked_bool_canonical_into[(1000, 10)]	45.4 µs	30.4 µs	+49.27%
⚡	Simulation	baseline_eq[4, 65536]	237.8 µs	185 µs	+28.54%
⚡	Simulation	baseline_lt[16, 65536]	275.4 µs	217.5 µs	+26.63%
⚡	Simulation	baseline_lt[4, 65536]	253 µs	200.8 µs	+26.01%
⚡	Simulation	chunked_varbinview_canonical_into[(1000, 10)]	197.7 µs	161.4 µs	+22.44%
⚡	Simulation	chunked_varbinview_into_canonical[(1000, 10)]	212.3 µs	176 µs	+20.64%
⚡	Simulation	baseline_eq[16, 65536]	260.2 µs	229.8 µs	+13.23%

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 polarsignals:trace-vortex-io (a7fba5f) with develop (7a53ad5)}