Support file stats on nested fields

[asubiotto]

Discussed in #5917

Problem

Currently, only top-level field stats are stored in the vortex file footer:

vortex/vortex-layout/src/layouts/file_stats.rs

Line 45 in 191228e

/// Note: for now this only collects top-level struct fields.

While chunk-level pruning on nested fields already works, file-level stats are missing, preventing whole-file pruning on nested fields or other optimizations. For example, we have some queries that can 100% be satisfied just by nested file stats, removing extra object storage reads.

Proposed approach

Store stats via post-order DType walk. Each leaf and nullable struct gets a stats set entry:

{name=utf8?, age=i32} → [name stats_set, age stats_set]
{name=utf8?, age=i32}? → [name stats_set, age stats_set, struct stats_set (null count)]

Extend the stats set flatbuffer with an is_nested: bool = false field for backward compatibility. When false, existing behavior is preserved; when true, entries follow the post-order walk.

Lists: For list(T), store stats only for the element column.

[asubiotto]

cc @joseph-isaacs