Allow omitting sum from default zone stats

[gatesn]

Summary

• add a writer-policy switch for omitting Sum from the default zoned aggregate set
• preserve the existing default behavior
• cover the disabled policy with a focused zoned-writer test

Why

The SQL-style Sum partial introduced in #9113 is a struct. Readers released before that change ignore the new aggregate option, infer the historical scalar Sum partial, and cannot deserialize newly written zone maps. Downstream systems with a forward-compatibility contract need a way to retain the new runtime Sum semantics while temporarily omitting the incompatible persisted optimization.

This option affects only default zone statistics. Explicit aggregate_fns remain authoritative, and data values and query-time Sum behavior are unchanged.

Validation

• cargo +nightly fmt --all
• cargo test -p vortex-layout default_aggregates --all-features
• cargo check -p vortex-file --all-features
• cargo clippy -p vortex-layout -p vortex-file --all-targets --all-features

Authored by Codex on behalf of Nicholas Gates.

[codspeed-hq]

Merging this PR will degrade performance by 45.69%

❌ 1 regressed benchmark
✅ 1943 untouched benchmarks
⏩ 85 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	decompress[u64, (1000, 16)]	72.2 µs	132.9 µs	-45.69%

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 ngates/forward-compatible-sum-zone-stats (ac7fc7a) with develop (5794e7d)}

Footnotes

1. 85 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. ↩