feat: factorise the joint once with History::joint

`posterior_of`, `posterior_of_at` and `expected_variance_reduction` each
built the joint precision matrix, factorised it, asked one question and
threw it away. The factorisation is O(n^3) in the history's appearances
and depends only on the fit, so a caller asking about every pair in a
standings table, every cell in a grid, or every candidate in an
active-learning sweep paid for the same factorisation once per question.

`History::joint()` returns a `Joint` handle that pays it once. Measured
on 1976 appearances, 90 queries: 68.4s one-shot against 745ms factorise
plus 93ms of queries — 81.6x, with bit-identical answers. Per query,
Criterion at 480 appearances: 9.0ms one-shot against 48us cached, 187x.

The handle borrows the history, which is what makes it correct with no
invalidation logic: the borrow checker forbids adding events or refitting
while it is alive, so there is no window in which the factorisation could
describe a fit that no longer exists. It also makes the lifetime of the
n^2 factor explicit rather than parking it in the history forever — at
4000 appearances that is 128MB, which is not something to cache silently.

Every question the joint answers turns out to be a bilinear form,

    c^T A^-1 a = (L^-1 c) . (L^-1 a)

so no caller ever needs L^-1 c itself. Replacing the general solve with a
forward substitution drops the back substitution as wasted work, halving
a query, and removes a failure mode: a variance as `c . (A^-1 c)` is a
difference of products that can round negative, where `|L^-1 c|^2` is a
sum of squares and cannot.

The one-shot calls are unchanged in cost and now delegate to the handle,
so the two paths cannot drift apart. tests/joint_handle.rs asserts they
agree bit for bit, including at pinned times, under UnknownKeys::Prior,
and across candidate matchups.

Refs #51

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

src/lib.rs

@@ -141,7 +141,7 @@ pub use event::{Event, Member, Team};
 pub use event_builder::EventBuilder;
 pub use game::{Game, GameOptions, OwnedGame};
 pub use gaussian::Gaussian;
-pub use history::{History, HistoryBuilder};
+pub use history::{History, HistoryBuilder, Joint};
 pub use key_table::KeyTable;
 use matrix::Matrix;
 pub use observer::{NullObserver, Observer};