Use aggregate descriptors for zoned stats

[gatesn]

Summary

• store zoned stats as aggregate-function partials keyed by AggregateFnRef display descriptors
• add magic-prefixed protobuf zoned metadata with legacy Stat bitset decoding
• lower StatFn against descriptor-named zone columns with legacy null/nan count bridges preserved

Checks

• cargo build -p vortex-layout
• cargo build -p vortex-tui
• cargo test -p vortex-layout zoned
• cargo test -p vortex-array stat_expr
• cargo test -p vortex-array min_
• cargo test -p vortex-array max_
• cargo test -p vortex-sqllogictest --test sqllogictests
• cargo +nightly fmt --all
• ./scripts/public-api.sh
• cargo clippy --all-targets --all-features
• git diff --check

[codspeed-hq]

Merging this PR will not alter performance

⚠️ 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.

✅ 1216 untouched benchmarks

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_{Comparing ngates/stats-7707/aggregate-zoned-stats (8b3da0d) with ngates/stats-7707/zoned-statfn-pruning (096128a)}