fix!: reject malformed games at the Game boundary too

I fixed this at `History`'s ingestion chokepoint and said the boundary
was complete. It was not. `Game` is a separate public entry point that
does not pass through that chokepoint, and every one of the same four
defects was still live there:

  Game::ranked(&[&[a]], ..)  -> PANIC at src/game.rs:317
  Game::scored(&[&[a]], ..)  -> PANIC at src/game.rs:317
  Game::ranked(&[&[], &[a]]) -> Ok, finite posterior for the opponent
  Game::scored(.., [NaN, 1]) -> Ok

The same panic, from safe API, in release. Fixing one path and
generalising from it is exactly the mistake that produced the
latest-slice joint bug: validating on the shape that cannot expose the
problem, then reporting the property as held.

`Game::validate_teams` is shared by `ranked` and `scored`, with the
non-finite score check in `scored` alongside it. Ranks need no equivalent
— they are `u32`.

`one_v_one` and `free_for_all` build their teams internally and are
unaffected; a test asserts all three well-formed constructors still
succeed, so the check cannot quietly widen.

BREAKING CHANGE: `Game::ranked` and `Game::scored` return
`NotEnoughTeams`, `EmptyTeam` or `InvalidParameter` for inputs they
previously panicked on or silently accepted.

Refs #18, #26

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

tests/game.rs

@@ -138,3 +138,115 @@ fn one_v_one_honours_convergence_options() {
     let (a_post, _) = Game::<i64, _>::one_v_one(&a, &b, Outcome::winner(0, 2), &options).unwrap();
     assert!(a_post.mu() > 25.0);
 }
+
+/// `Game` is a public entry point that does not pass through `History`'s
+/// ingestion chokepoint, so it needs its own boundary — and did not have one.
+///
+/// A one-team game panicked at `src/game.rs:317` with "range start index 1 out
+/// of range for slice of length 0", in release, from safe API. This is the
+/// same defect `tests/ingestion_shape.rs` covers for `History`; fixing that
+/// path left this one open, because they share no validation.
+mod malformed_games {
+    use super::*;
+
+    #[test]
+    fn a_one_team_ranked_game_is_an_error_not_a_panic() {
+        let a = default_rating();
+        let err = Game::<i64, _>::ranked(&[&[a]], Outcome::winner(0, 1), &GameOptions::default())
+            .unwrap_err();
+        assert!(
+            matches!(err, InferenceError::NotEnoughTeams { got: 1 }),
+            "{err:?}"
+        );
+    }
+
+    #[test]
+    fn a_one_team_scored_game_is_an_error_not_a_panic() {
+        let a = default_rating();
+        let err = Game::<i64, _>::scored(
+            &[&[a]],
+            Outcome::scores([1.0]),
+            &GameOptions {
+                score_sigma: 1.0,
+                ..GameOptions::default()
+            },
+        )
+        .unwrap_err();
+        assert!(
+            matches!(err, InferenceError::NotEnoughTeams { got: 1 }),
+            "{err:?}"
+        );
+    }
+
+    #[test]
+    fn a_zero_team_game_is_an_error() {
+        let err =
+            Game::<i64, ConstantDrift>::ranked(&[], Outcome::ranking([]), &GameOptions::default())
+                .unwrap_err();
+        assert!(
+            matches!(err, InferenceError::NotEnoughTeams { got: 0 }),
+            "{err:?}"
+        );
+    }
+
+    /// The quiet half: an empty team contributed no performance, so the game
+    /// returned a finite posterior for its opponent as though it had won one.
+    #[test]
+    fn an_empty_team_is_an_error() {
+        let a = default_rating();
+        let err =
+            Game::<i64, _>::ranked(&[&[], &[a]], Outcome::winner(0, 2), &GameOptions::default())
+                .unwrap_err();
+        assert!(
+            matches!(err, InferenceError::EmptyTeam { team: 0 }),
+            "{err:?}"
+        );
+    }
+
+    #[test]
+    fn a_non_finite_score_is_an_error() {
+        let a = default_rating();
+        for bad in [f64::NAN, f64::INFINITY, f64::NEG_INFINITY] {
+            let err = Game::<i64, _>::scored(
+                &[&[a], &[a]],
+                Outcome::scores([bad, 1.0]),
+                &GameOptions {
+                    score_sigma: 1.0,
+                    ..GameOptions::default()
+                },
+            )
+            .unwrap_err();
+            assert!(
+                matches!(err, InferenceError::InvalidParameter { name: "score", .. }),
+                "{bad}: {err:?}"
+            );
+        }
+    }
+
+    /// `free_for_all` and `one_v_one` build their teams internally, so they
+    /// must keep working — the check must not catch well-formed games.
+    #[test]
+    fn well_formed_games_are_untouched() {
+        let a = default_rating();
+        assert!(
+            Game::<i64, _>::ranked(
+                &[&[a], &[a]],
+                Outcome::winner(0, 2),
+                &GameOptions::default()
+            )
+            .is_ok()
+        );
+        assert!(
+            Game::<i64, _>::free_for_all(
+                &[&a, &a, &a],
+                Outcome::ranking([0, 1, 2]),
+                &GameOptions::default()
+            )
+            .is_ok()
+        );
+        assert!(
+            Game::<i64, _>::one_v_one(&a, &a, Outcome::winner(0, 2), &GameOptions::default())
+                .is_ok()
+        );
+    }
+}