refactor!: the joint is reached through Joint, not mirrored on History

`posterior_of`, `posterior_of_at` and `expected_variance_reduction`
existed twice: once on `Joint`, and once on `History` as one-shot
wrappers whose whole body was `self.joint()?.<same>(..)`.

The wrappers re-factorised on every call — their own docs said so,
warning the reader to take a `Joint` instead — and they were what
smuggled the scored-only precondition onto the flat surface. A user
following the quickstart builds a ranked history, sees `posterior_of` in
the method list, and it never works. `h.joint()?.posterior_of(..)` is
one call longer and tells the truth: you need a joint, and a joint needs
a scored history.

That leaves three tiers instead of a flat surface with a hidden
precondition: `History` fits and reads, `predict_*` forecasts, `Joint`
answers exact joint questions.

`predict_margin` was itself calling `self.posterior_of`; it goes through
`self.joint()?` directly now.

The `Joint` methods' docs referred back to the wrappers for their real
content ("Identical to `History::posterior_of`, without re-paying the
factorisation"), so they now carry it: what a linear functional means,
which appearance each competitor is read at, and why
`expected_variance_reduction` belongs on the handle.

`tests/joint_handle.rs` had three tests comparing the wrapper against
the handle. That comparison is gone, but the property behind it is not —
they now compare a *reused* joint against a *fresh* one per question,
which is the actual correctness claim behind caching the factorisation
(#51), without the wrapper in the middle.

Closes #78.

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

tests/marginal_calibration.rs

@@ -281,6 +281,8 @@ fn posterior_of_matches_the_exact_joint() {
 
         for (i, j) in [(0usize, 1usize), (0, 2), (1, 3), (2, 4)] {
             let got = h
+                .joint()
+                .unwrap()
                 .posterior_of(&[(&key(i), 1.0), (&key(j), -1.0)])
                 .expect("scored slice should have a joint");
             let exact_sd = (cov[i][i] + cov[j][j] - 2.0 * cov[i][j]).sqrt();
@@ -302,7 +304,7 @@ fn posterior_of_matches_the_exact_joint() {
 
         // A single competitor: this is where the loopy marginal was 2x narrow.
         for (i, row) in cov.iter().enumerate() {
-            let got = h.posterior_of(&[(&key(i), 1.0)]).unwrap();
+            let got = h.joint().unwrap().posterior_of(&[(&key(i), 1.0)]).unwrap();
             let exact_sd = row[i].sqrt();
             assert!(
                 (got.sigma() - exact_sd).abs() / exact_sd < 1e-9,
@@ -361,6 +363,8 @@ fn cost_scaling() {
 
         let t = Instant::now();
         let g = h
+            .joint()
+            .unwrap()
             .posterior_of(&[(&names[0], 1.0), (&names[1], -1.0)])
             .unwrap();
         println!("  n={n:>4}: {:>10.2?}  sigma {:.6}", t.elapsed(), g.sigma());