Is your feature request related to a problem?

After the arrow-rs migration, ExtensionArray is DataArray<ExtensionType> which treats it as a physical type. But there's no DataType::Extension in arrow-rs — extension types are a logical concept (metadata over a storage type). The current representation doesn't reflect this.

Describe the solution you'd like

Refactor ExtensionArray from DataArray<ExtensionType> to a custom struct backed by Series:

pub struct ExtensionArray {
    pub field: Arc<Field>,
    pub physical: Series, // the storage-typed data
}

Operations delegate to the inner Series, which handles runtime dispatch to the correct physical type. ExtensionType becomes non-physical (removed from DaftPhysicalType/DaftArrowBackedType), and DataType::Extension is added to is_logical().

Describe alternatives you've considered

Using LogicalArrayImpl<ExtensionType> with a fixed physical type, but Extension's storage type varies per instance (could be Int64, Utf8, FixedSizeList, etc.), which breaks the compile-time fixed PhysicalType requirement.

Additional Context

Prerequisite work for the public ExtensionType API (#6396). Casting to/from extension types is already implemented on this branch.