Internal helpers for the vecshift v2 input contract

These helpers centralize the validation that every public entry point consuming vecshift output performs. They check that the input is a data.table, that it carries the columns the caller actually needs, and (when vecshift >= 2.0.0 is installed) that the object is a vecshift_result produced with day-level granularity. Class and metadata are reattached to consolidation outputs so downstream code keeps seeing the same S3 type.