refactor!: typed discriminators for InferenceError

Six of fifteen variants carried a `&'static str` discriminator, about
thirty magic strings between them, and the only thing a caller could do
with one was print it. Four new enums replace them:

    Parameter        13 variants, replacing 9 strings in InvalidParameter
    Shape             4 variants, replacing 10 in MismatchedShape
    OutcomeKind       2 variants, replacing WrongOutcomeKind's three fields
    CompetitorField   2 variants, replacing ConflictingCompetitorConfig's

`InvalidProbability` folds into `InvalidParameter` as
`Parameter::PDraw`. It was a bespoke variant for one scalar while every
other scalar shared `InvalidParameter`, and it omitted the parameter
name — so the same parameter had two mechanisms.

`JointUnavailable { reason: &'static str }` splits into `EmptyHistory`,
`JointRequiresScoredEvents` and `NotPositiveDefinite`. The three are
conditions a caller branches on differently — add events, use
`predict_win_probabilities`, or reconsider the priors — and telling them
apart used to mean string-matching English. One test already proved the
distinction was load-bearing: the blanket conversion mapped the
empty-history case onto the ranked one and `an_empty_history_has_no_joint`
caught it immediately.

`NonFiniteResult` splits into `NonFiniteStep { context, step }` and
`NonFiniteSkill { mu, sigma }`. One `step: (f64, f64)` field was
carrying a sweep step from `converge` and a skill's own moments from a
prediction — two situations in one variant, and a field name that could
only be right for one of them.

`InvalidParameter { name: "beta with point-mass skills" }` becomes
`NoPerformanceVariance`. It was never a parameter out of range: both
values are individually valid and it is their combination that leaves
nothing varying.

Three `Display` impls did not meet the standard the others set, and the
typed data is what makes fixing them possible:

    before  drift variance is invalid: NaN
    after   drift variance must be finite and non-negative (got NaN)

    before  kinds: expected length 3, got 2
    after   the outcome describes a different number of teams than the
            event has: expected 3, got 2

    before  Game::ranked: expected Outcome::Ranked, got Outcome::Scored
    after   expected Outcome::Ranked, got Outcome::Scored; call
            Game::scored for a scored outcome

`Parameter::range()` states each parameter's actual bounds, which no
`&'static str` name could have. `error::message_tests` renders every one
and asserts each is a sentence rather than a label, and that the three
above now carry a range or a next step.

The four internal `MismatchedShape` kinds — `results`, `times`, `kinds`,
and the weights array — collapse to `Shape::Internal`, whose `Display`
says plainly that reaching it is a bug in this crate. They are checks on
`add_events_with_prior`'s own parallel arrays and are unreachable
through the public API; they stay checked rather than becoming
`debug_assert!`s, because release is where this crate's defects hide.

Closes #74.

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

src/acquisition.rs

@@ -123,7 +123,7 @@ fn u_minus_ln1p(u: f64) -> f64 {
 /// - `EmptyTeam` if any team has no members.
 /// - `TooManyTeams` if the outcome space is too large to enumerate; see
 ///   [`MAX_PREDICTED_TEAMS`](crate::MAX_PREDICTED_TEAMS).
-/// - `InvalidProbability` if `options.p_draw` is outside `[0.0, 1.0)`.
+/// - `InvalidParameter` for a `p_draw` outside `[0.0, 1.0)`.
 /// - `GridTooCoarse` when the performance sigmas are too far apart to
 ///   integrate on one grid. This comes from `outcome_distribution`, which runs
 ///   before any inference — so it is not covered by "anything `Game::ranked`
@@ -144,7 +144,8 @@ pub fn expected_information_gain<T: Time, D: Drift<T>>(
         });
     }
     if !(0.0..1.0).contains(&options.p_draw) {
-        return Err(InferenceError::InvalidProbability {
+        return Err(InferenceError::InvalidParameter {
+            parameter: crate::Parameter::PDraw,
             value: options.p_draw,
         });
     }
@@ -367,7 +368,10 @@ mod tests {
         ));
         assert!(matches!(
             expected_information_gain(&[&a, &a], &options(1.5)),
-            Err(InferenceError::InvalidProbability { .. })
+            Err(InferenceError::InvalidParameter {
+                parameter: crate::Parameter::PDraw,
+                ..
+            })
         ));
     }