bug: Flatten panics on a multivector batch without a row-id column

Description

Flatten::transform in rust/lance-index/src/vector/transform.rs flattens a List<FixedSizeList> column by repeating each row id once per inner vector, and it reads the row ids with the indexing operator:

DataType::List(_) => {
    let row_ids = batch[ROW_ID].as_primitive::<UInt64Type>();

RecordBatch's Index impl panics when the column is absent, so a multivector batch without _rowid takes the caller down instead of returning an error. Flatten is public with a public constructor, and the fixed-size-list branch immediately above returns the batch untouched without needing the column, so nothing in the type or the contract says the column has to be there.

The other transformers in this file report their missing or mistyped columns as errors, and rust/AGENTS.md asks for column_by_name() in production code with batch["column"] left to tests.

Expected behavior

Return an error naming the missing column, like the neighbouring transformers.

Lance version

13.0.0-beta.4 (main)

Language binding

Rust

Start in rust/lance-index/src/vector/transform.rs at Flatten::transform, then inspect the neighboring transformers and rust/AGENTS.md for the expected column access pattern. Reproduce a List batch without _rowid and verify that the transform returns an error naming the missing column instead of panicking.