Fix list-of-struct nested projection

[godnight10061]

Summary

• Add a component-shredded vortex.list layout for List/FixedSizeList columns
• Enable nested projection/pushdown for list<struct> (e.g. projecting items.a)
• Extend vortex.get_item and the list reader to support list<struct>

Why

• Today list columns are written as opaque blobs, which prevents nested projections from being pushed down and breaks list-of-struct field selection.

Tests

• cargo +nightly fmt --all --check
• cargo clippy --locked --workspace --all-features --all-targets --exclude vortex-bench --exclude vortex-python --exclude vortex-duckdb --exclude vortex-fuzz --exclude duckdb-bench --exclude lance-bench --exclude datafusion-bench --exclude random-access-bench --exclude compress-bench -- -D warnings
• cargo nextest run --locked --workspace --all-features --no-fail-fast --exclude vortex-bench --exclude vortex-python --exclude vortex-duckdb --exclude vortex-fuzz --exclude duckdb-bench --exclude lance-bench --exclude datafusion-bench --exclude random-access-bench --exclude compress-bench (fails locally: vortex-cuda for_::tests::test_cuda_for_decompression requires a CUDA driver)
• cargo nextest run --locked --workspace --all-features --no-fail-fast --exclude vortex-cuda --exclude vortex-bench --exclude vortex-python --exclude vortex-duckdb --exclude vortex-fuzz --exclude duckdb-bench --exclude lance-bench --exclude datafusion-bench --exclude random-access-bench --exclude compress-bench

Compatibility

• New files may be written with a new list layout encoding id (vortex.list). Older readers that don?t know this encoding won?t be able to read those files; new readers continue to read existing files.

References

• Part of Better data layout for nested/repeated schemas #4889

[godnight10061]

I wasn't able to apply labels from a fork (permission denied). Could a maintainer please add the ix label for the changelog/CI gating?