refactor!: K comes first in History, HistoryBuilder and Joint

`K` is the one type parameter people change, and it was last. Naming a
history in a struct field meant writing all four to say one thing:

    struct Ladder { history: History<i64, ConstantDrift, NullObserver, String> }
    struct Analysis<'h> { joint: Joint<'h, i64, ConstantDrift, NullObserver, &'static str> }

Now:

    struct Ladder { history: History<String> }
    struct Analysis<'h> { joint: Joint<'h> }

`History<K, T, D, O>`, all four defaulted. Bounds may reference later
parameters, so `D: Drift<T> = ConstantDrift` is legal in third position.
`Joint` gains the same defaults, so `Joint<'h, String>` spells it.

72 call sites swapped, and the reorder makes most of them shorter: 18
now read `History<String>` and the `&'static str` ones read `History`.
The two turbofished builders shrink from
`HistoryBuilder::<Untimed, _, _, String>::new()` to
`HistoryBuilder::<String, Untimed>::new()`.

`Joint` keeps `O` structurally, defaulted rather than removed. #72 notes
it never touches the observer, which is true — but it borrows the whole
`&'h History<K, T, D, O>` and calls `History::resolve_terms`, so dropping
the parameter means either a view type or moving that method off
`History`. The default already buys the entire user-visible benefit,
which was the spelling.

Refs #72.

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

tests/predict_margin.rs

@@ -6,9 +6,7 @@ use trueskill_tt::{
     UnknownKeys,
 };
 
-fn builder(
-    policy: UnknownKeys,
-) -> History<i64, ConstantDrift, trueskill_tt::NullObserver, &'static str> {
+fn builder(policy: UnknownKeys) -> History {
     History::builder()
         .mu(0.0)
         .sigma(6.0)
@@ -37,9 +35,7 @@ fn round(a: &'static str, b: &'static str, sa: f64, sb: f64) -> Event<i64, &'sta
 
 /// A history where "veteran" and "regular" are well observed and "novice"
 /// appears once.
-fn fitted(
-    policy: UnknownKeys,
-) -> History<i64, ConstantDrift, trueskill_tt::NullObserver, &'static str> {
+fn fitted(policy: UnknownKeys) -> History {
     let mut h = builder(policy);
     let mut events: Vec<_> = (0..40)
         .map(|t| round("veteran", "regular", 10.0 + f64::from(t % 3), 5.0))