Closes #198

Picks up where #209 left off with @Fokko's feedback addressed. Thanks @jayceslesar for the original work.

Rationale for this change

Adds CAST(column AS type) parsing that maps to Iceberg transforms (date → DayTransform, year → YearTransform, month → MonthTransform, hour → HourTransform). Also implements BoundTransform.ref() and eval() — both were missing abstract methods required by BoundTerm. Introduces UnboundTransform with bind() that validates via can_transform().

Are these changes tested?

Yes. 11 new tests covering all transform types, case insensitivity, nested fields, unsupported types, comparison operators, and boolean composition.

Are there any user-facing changes?

No.