Fix flat reader subrange decode reuse

[lukekim]

Fixes #8587.

Summary

• Memoize FlatReader's decoded array future so synthetic subrange scans share the decoded flat segment instead of issuing repeated decode work.
• Add regression coverage for the query patterns that regressed after perf(scan): intra-file decode parallelism — sub-split large chunk spans #8400, including projection-only, filter-only, filtered projection, computed projection, string filtered projection, and string filtered computed projection cases.

Validation

• cargo nextest run -p vortex-layout -E 'test(layouts::flat::reader)'
• cargo nextest run -p vortex-layout
• cargo clippy -p vortex-layout --all-targets --all-features
• git diff --check
• cargo bench --workspace
• Re-ran SQL benches with /opt/homebrew/bin/uv 0.11.24; core suites passed: Appian, TPCH, TPCDS, ClickBench, ClickBench sorted, FineWeb, and GH Archive via direct binary rerun. PolarSignals/StatPopGen exposed pre-existing benchmark-definition/runtime backend failures, and bare Public BI requires --opt dataset=<name>.