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/marginal_calibration.rs

@@ -132,10 +132,8 @@ fn key(i: usize) -> &'static str {
 }
 
 /// Returns (worst mean error, worst sd ratio).
-fn fitted(
-    obs: &[(usize, usize, f64)],
-) -> History<i64, ConstantDrift, trueskill_tt::NullObserver, &'static str> {
-    let mut h: History<i64, _, _, &'static str> = History::builder()
+fn fitted(obs: &[(usize, usize, f64)]) -> History {
+    let mut h: History = History::builder()
         .mu(MU0)
         .sigma(SIGMA0)
         .beta(BETA)
@@ -324,7 +322,7 @@ fn cost_scaling() {
     use std::time::Instant;
     for n in [50usize, 100, 200, 400, 800] {
         let names: Vec<String> = (0..n).map(|i| format!("c{i}")).collect();
-        let mut h: History<i64, _, _, String> = History::builder()
+        let mut h: History<String> = History::builder()
             .key_type::<String>()
             .score_sigma(2.0)
             .drift(ConstantDrift::new(0.0))