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/key_ergonomics.rs

@@ -8,10 +8,10 @@
 //! Both key types are exercised in every test, because the point is that the
 //! spelling is the same.
 
-use trueskill_tt::{ConstantDrift, History, NullObserver};
+use trueskill_tt::{ConstantDrift, History};
 
-type Owned = History<i64, ConstantDrift, NullObserver, String>;
-type Borrowed = History<i64, ConstantDrift, NullObserver, &'static str>;
+type Owned = History<String>;
+type Borrowed = History;
 
 fn owned() -> Owned {
     let mut h: Owned = History::builder().key_type::<String>().build();