## Todo 2 — news-core domain types

Command: `cargo test -p news-core`

```
$ cargo test -p news-core
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.03s
     Running unittests src/lib.rs (target/debug/deps/news_core-73165881e402db9b)

running 2 tests
test model::tests::opinion_item_serializes_kind_as_snake_case_discriminant ... ok
test model::tests::round_trip ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests news_core

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
```

- `round_trip` covers all 5 types (`Source`, `RawItem`, `StoryCluster`,
  `ScoreBreakdown`, `Feedback`): serialize with `serde_json::to_string`,
  deserialize back, `assert_eq!(decoded, original)` via `pretty_assertions`.
- `opinion_item_serializes_kind_as_snake_case_discriminant` locks the wire
  format: a `RawItem` with `kind: ItemKind::Opinion` serializes the
  discriminant as the literal string `"opinion"`.

Also verified clean (not required by acceptance criteria but run as part of
the post-write review loop):

```
$ cargo fmt --all --check        # exit 0
$ cargo clippy -p news-core --all-targets -- -D warnings   # exit 0, no warnings
$ cargo build --workspace        # exit 0, all 6 crates compile
```