Function plain_encoded_byte_size 

fn plain_encoded_byte_size<T: DataType>(value: &T::T) -> usize

Plain-encoded byte cost of a single value of type T::T.

Derived from ParquetValueType::dict_encoding_size (which returns (per-value overhead, value-bytes)) so we don’t add a parallel per-value-size hook to the trait. Mirrors the dispatch in KeyStorage::push (encodings/encoding/dict_encoder.rs).

Placed at the end of the module deliberately. Inserting it above the ColumnValueEncoder trait shifts the trait and ColumnValueEncoderImpl within the compiled module enough to perturb downstream code placement, which measurably regresses unrelated arrow-writer string benchmarks (~5-9% on string / string_and_binary_view). Defining it last keeps the hot encoder code at the offsets it has on main.