Enum variant payload types were silently dropped — `struct_item` and
`trait_item` had type-reference handlers but `enum_item` had none, so the
variant field types were never traversed.
Add an `enum_item` branch that walks
`enum_variant_list -> enum_variant -> ordered_field_declaration_list`
(tuple variants, `Click(Logger)`) and `field_declaration_list` (struct
variants, `Resize { size: Dim }`), emitting a `references` edge from the enum
to each field type. Reuses the same type collection as the struct path. Adds
an enum to the fixture and a regression test.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
