refactor!: scores_with_noise, and History::quality

Two names that described the wrong thing.

`scores_with_sigma(scores, sigma)` reads as "these scores have prior
sigma 2.0". The quantity is observation noise on the score *margin*, in
the units of the scores, and it is spelled `score_sigma` at every config
site — `HistoryBuilder::score_sigma`, `GameOptions::score_sigma`,
`EventKind::Scored { score_sigma }` — so this was the one place the
crate used a third meaning of "sigma" for it. Its own doc had to
disambiguate itself: "`sigma` overrides `HistoryBuilder::score_sigma`".
`scores_with_noise(scores, score_sigma)` on both `Outcome` and
`EventBuilder`.

`predict_quality` predicts nothing. Its own doc says it answers "is this
matchup *fair*", not "what will happen", and the `predict_*` family is
otherwise exactly the methods returning a probability or a distribution
over outcomes. `History::quality` also makes the free/method pair
consistent: free `quality` pairs with `History::quality` the way free
`expected_information_gain` already pairs with
`History::expected_information_gain`. The rule that was already being
followed and never stated — a free function scores a hypothetical from
explicit parameters, the same-named method asks it against the fit — is
now written on the method.

Closes #75. Refs #78 (part 4).

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

tests/api_shape.rs

@@ -203,7 +203,7 @@ fn predict_quality_two_teams() {
     h.record_winner(&"a", &"b", 1).unwrap();
     let _ = h.converge().unwrap();
 
-    let q = h.predict_quality(&[&[&"a"], &[&"b"]]).unwrap();
+    let q = h.quality(&[&[&"a"], &[&"b"]]).unwrap();
     assert!(q > 0.0 && q <= 1.0);
 }
 

tests/constructor_validation.rs

@@ -102,7 +102,7 @@ fn every_magnitude_parameter_rejects_a_negative_value() {
             }),
         ),
         (
-            "Outcome::scores_with_sigma (at ingestion)",
+            "Outcome::scores_with_noise (at ingestion)",
             Box::new(|v| {
                 let mut h = History::builder().build();
                 h.add_events(vec![trueskill_tt::Event {
@@ -111,7 +111,7 @@ fn every_magnitude_parameter_rejects_a_negative_value() {
                         trueskill_tt::Team::with_members([Member::new("a")]),
                         trueskill_tt::Team::with_members([Member::new("b")]),
                     ],
-                    outcome: Outcome::scores_with_sigma([3.0, 1.0], v),
+                    outcome: Outcome::scores_with_noise([3.0, 1.0], v),
                 }])
                 .is_err()
             }),

tests/degenerate_inputs.rs

@@ -222,7 +222,7 @@ fn scored_event_rejects_non_positive_sigma() {
         .event(1)
         .team(["a"])
         .team(["b"])
-        .scores_with_sigma([3.0, 1.0], f64::NAN)
+        .scores_with_noise([3.0, 1.0], f64::NAN)
         .commit()
         .unwrap_err();
     assert!(matches!(

tests/key_ergonomics.rs

@@ -52,7 +52,7 @@ fn every_team_shaped_query_accepts_the_same_slice() {
     let h = owned();
     let teams: &[&[&str]] = &[&["alice"], &["bob"]];
 
-    h.predict_quality(teams).expect("quality");
+    h.quality(teams).expect("quality");
     let _ = h.predict_outcome(teams).expect("outcome");
     h.predict_ranking(teams, &[0, 1]).expect("ranking");
     h.expected_information_gain(teams)

tests/prediction.rs

@@ -34,7 +34,7 @@ fn unknown_keys_are_reported_not_silently_dropped() {
         h.predict_win_probabilities(&[&[&"a"], &[&"ghost"]])
             .is_err()
     );
-    assert!(h.predict_quality(&[&[&"a"], &[&"ghost"]]).is_err());
+    assert!(h.quality(&[&[&"a"], &[&"ghost"]]).is_err());
     assert!(h.predict_ranking(&[&[&"a"], &[&"ghost"]], &[0, 1]).is_err());
 }
 
@@ -407,7 +407,7 @@ fn prior_reaches_every_prediction_entry_point() {
     let h = history_with_policy(&["a", "b"], trueskill_tt::UnknownKeys::Prior);
     let teams: &[&[&&str]] = &[&[&"a"], &[&"ghost"]];
 
-    assert!(h.predict_quality(teams).is_ok());
+    assert!(h.quality(teams).is_ok());
     assert!(h.predict_win_probabilities(teams).is_ok());
     assert!(h.predict_outcome(teams).is_ok());
     assert!(h.predict_ranking(teams, &[0, 1]).is_ok());

tests/prediction_guards.rs

@@ -78,7 +78,7 @@ macro_rules! all_predictions {
     ($h:ident, $f:expr) => {{
         let teams: &[&[&&'static str]] = &[&[&"a"], &[&"b"]];
         let f = $f;
-        f("predict_quality", $h.predict_quality(teams).map(|_| ()));
+        f("quality", $h.quality(teams).map(|_| ()));
         f(
             "predict_win_probabilities",
             $h.predict_win_probabilities(teams).map(|_| ()),
@@ -116,7 +116,7 @@ fn degenerate_performances_are_refused_rather_than_answered_wrongly() {
     assert_eq!(skill.sigma(), 0.0);
     assert!(skill.mu().is_finite());
 
-    // `predict_quality` previously PANICKED here, out of a method that returns
+    // `quality` previously PANICKED here, out of a method that returns
     // `Result`: the contrast covariance is exactly singular when beta is zero
     // and every skill is a point mass.
     all_predictions!(h, |name: &str, r: Result<(), InferenceError>| {

tests/quality.rs

@@ -110,11 +110,8 @@ fn history_predict_quality_supports_three_teams() {
     h.record_winner(&"b", &"c", 2).unwrap();
     let _ = h.converge().unwrap();
 
-    let q = h.predict_quality(&[&[&"a"], &[&"b"], &[&"c"]]).unwrap();
-    assert!(
-        q.is_finite(),
-        "3-team predict_quality must be finite, got {q}"
-    );
+    let q = h.quality(&[&[&"a"], &[&"b"], &[&"c"]]).unwrap();
+    assert!(q.is_finite(), "3-team quality must be finite, got {q}");
     assert!((0.0..=1.0).contains(&q), "out of range: {q}");
 }
 

tests/validation.rs

@@ -139,7 +139,7 @@ fn ingestion_rejects_a_tie_without_a_draw_probability() {
     );
 }
 
-/// `Outcome::scores_with_sigma` documents that a non-positive sigma is
+/// `Outcome::scores_with_noise` documents that a non-positive sigma is
 /// accepted at construction and rejected at ingestion.
 #[test]
 fn ingestion_rejects_a_non_positive_per_event_score_sigma() {
@@ -152,7 +152,7 @@ fn ingestion_rejects_a_non_positive_per_event_score_sigma() {
                     Team::with_members([Member::new("a")]),
                     Team::with_members([Member::new("b")]),
                 ],
-                outcome: Outcome::scores_with_sigma([21.0, 9.0], sigma),
+                outcome: Outcome::scores_with_noise([21.0, 9.0], sigma),
             }])
             .expect_err("a non-positive per-event sigma must be rejected");
         assert!(