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/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()
             }),