feat(derive): add explicit nested field option

[bengreenier]

Summary

• add an explicit field option: #[partially(nested)]
• for nested fields, generate <FieldType as Partial>::Item in the partial struct
• for nested fields, generated apply_some recurses via Partial::apply_some
• keep nested behavior opt-in (no heuristic auto-detection)
• preserve precedence/validation with other field options (omit, transparent, as_type)
• add tests/docs/changelog updates for explicit nested behavior
• add regression coverage that non-annotated fields stay Option<T> and reject nested payloads at compile time

Testing

• cargo test -p partially_derive
• cargo test -p partially