fix(layout): preserve nullability of structs nested in structs

[miniex]

Summary

A nullable struct field nested directly inside another single-field struct came back non-nullable on a file round-trip ({c0={a={b=i32}?}?} was read back as {c0={a={b=i32}}?}), silently dropping the inner ?. The struct reader's single-partition fast path delegated this scope's reconstruction to the child, so chained single-field structs accumulated pack wrappers and each scope's validity landed at the wrong nesting level. I stopped the partition splitter from collapsing pack/merge reconstruction nodes, and made the struct reader take the fast path only when nothing is reconstructed at this level, so reconstruction now flows through the multi-partition path, one level per scope. Multi-field structs were already unaffected.

The fuzzer's struct-of-struct assume() guard can be dropped once the fuzzer is on develop.

Closes: #8348

Testing

Added a round-trip regression test in vortex-file/tests/test_write_table.rs for the nested nullable-struct schema; it dropped the inner ? before this change and now round-trips unchanged. cargo nextest run -p vortex-array -p vortex-layout -p vortex-file passes (3270 tests, including the partition and struct-reader regressions); cargo +nightly fmt --all --check and cargo clippy --all-targets --all-features are clean.

---

I'm Korean, so sorry if any wording reads a little awkward.

[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.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 3 improved benchmarks
❌ 1 regressed benchmark
✅ 1585 untouched benchmarks
⏩ 4 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_varbinview_into_canonical[(1000, 10)]	168.8 µs	205.6 µs	-17.89%
⚡	Simulation	chunked_varbinview_canonical_into[(100, 100)]	259.5 µs	224.4 µs	+15.64%
⚡	Simulation	chunked_varbinview_into_canonical[(100, 100)]	306.6 µs	271.7 µs	+12.84%
⚡	Simulation	bitwise_not_vortex_buffer_mut[128]	273.6 ns	244.4 ns	+11.93%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing miniex:fix/nested-struct-nullability (9b2114d) with develop (15cec3b)}

Footnotes

1. 4 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. ↩

[robert3005]

This might be required but I wonder if there's a more elegant solution to this? As in can we correctly handle pack/merge/get_item validity such that we account for validity when doing expression rewrite so the splitting ends up being correct.