bug(overlay): struct cell validity is not replaced by a data overlay

Summary

Data overlay resolution cannot change the validity (null-ness) of a struct cell. Overlaying a nullable struct's children updates the child values but silently preserves the base struct's null buffer, so NULL -> non-NULL and non-NULL -> NULL transitions are lost.

This contradicts the overlay contract, which states that a covered offset holding NULL overrides that cell to NULL (see rust/lance/src/dataset/overlay/writer.rs module docs and docs/src/format/table/data_overlay_file.md).

Cause

Overlay resolution is per atomic field, and a struct is recursed through rather than treated as an atomic unit — only its leaves are atomic (rust/lance/src/dataset/overlay.rs:371-379).

At merge time, splice_by_ids destructures the base StructArray and rebuilds it with the base null buffer:

// rust/lance/src/dataset/overlay.rs:566-575
let (fields, mut children, nulls) = structs.clone().into_parts();
children[child_pos] = splice_by_ids(...)?;
Ok(Arc::new(StructArray::try_new_with_length(
    fields, children, nulls, len,   // <- base `nulls`, never the overlay's
)?))

The writer does faithfully encode the struct's validity into the overlay value file; the read path structurally ignores it.

Reproduction

1. Create a two-row dataset with a nullable struct column, child values [1, 2], struct validity [false, true].
2. Overlay both rows with child values [10, 20] and struct validity [true, false].
3. Commit and scan.

Observed: child values [10, 20] (correct), struct validity [false, true] (stale).
Expected: struct validity [true, false].

Options

1. Represent and merge validity at every struct ancestor — either give struct parents their own atomic-field entry carrying only validity, or have the merge recompute parent nulls from covered children. Read-path change, possibly a format change.
2. Reject nullable struct ancestors at the writer until (1) lands — fail closed rather than returning wrong data.

Interim mitigation

Tracking this separately from https://github.com/lance-format/lance/pull/8761, which adds OverlayWriter. That PR takes approach (2) as a stopgap so the writer cannot produce an overlay whose semantics the read path will not honor. This issue tracks doing it properly.

Read rust/lance/src/dataset/overlay.rs, especially atomic-field resolution and splice_by_ids around lines 371-379 and 566-575, alongside the overlay contract in rust/lance/src/dataset/overlay/writer.rs and docs/src/format/table/data_overlay_file.md. Determine how struct validity should be represented or merged, then verify that the reproduction changes validity from [false, true] to [true, false] after scanning.