Support nested column references in UpdateBuilder::set()

UpdateBuilder::set() rejects any column name containing ., so a nested field cannot be updated directly — it can only be changed by setting its whole top-level struct column.

Location — rust/lance/src/dataset/write/update.rs, in set():

// TODO: support nested column references. This is mostly blocked on the
// ability to insert them into the RecordBatch properly.
if column.as_ref().contains('.') {
    return Err(Error::not_supported_source(
        format!(
            "Nested column references are not yet supported. Referenced: {}",
            column.as_ref(),
        )
        .into(),
    ));
}

Impact

• To change s.x, callers must SET s = <whole struct>, which is awkward and rewrites the entire struct value.
• This is what made #7412 necessary: a whole-struct update resolves to the parent field id, so a scalar index on a nested child field (s.x) had to be handled specially to avoid being wrongly extended over the rewritten fragment.

Ask

Support set("s.x", <expr>) for nested leaf columns. The existing TODO notes this is mostly blocked on inserting nested columns into the RecordBatch properly. Once supported, the index-coverage logic can target the leaf field directly instead of conservatively expanding the whole struct subtree.

Start in rust/lance/src/dataset/write/update.rs at UpdateBuilder::set() and trace how update expressions are inserted into a RecordBatch. Study the existing handling of whole-struct updates and the index-coverage logic described in the issue. Done means set("s.x", ) updates the nested leaf directly without rejecting the reference or rewriting the whole struct.