docs: correct drifted documentation and compile the README in CI

Four README code blocks no longer compiled: `Player` was renamed `Rating`
in T2, the `Drift` trait gained a `T: Time` parameter and a second method,
and two blocks were missing imports outright. The `Rating` example needed
more than a rename — with the binding unused, `T` is ambiguous because
`ConstantDrift` implements `Drift<T>` for every `T`, so it now carries an
explicit annotation.

Nothing compiled those blocks. `src/lib.rs` gains a `cfg(doctest)` struct
carrying `#[doc = include_str!("../README.md")]`, which turns every `rust`
block into a doctest without displacing the curated crate docs as the
front page. Verified it bites: reintroducing `Player` fails the build with
E0432 rather than shipping. Illustrative blocks are fenced `text` — note
that a bare fence defaults to `rust` under rustdoc, which is how the
`variance_delta = elapsed * γ²` formula became a compile error.

Prose fixes: README claimed `Gaussian::forget` takes a square root (it
works in variance space) and pointed at a `.gamma()` builder method that
does not exist. CLAUDE.md's data-flow diagram spliced the public ingestion
shape into the internal one — `Team` is not in that chain — listed
`cdf()`/`erfc()` as public when they are `pub(crate)` and private, and
called `SkillStore` public when only `CompetitorStore` escapes the crate.

Rustdoc fixes: `EventBuilder::scores_with_sigma` claimed a debug-assert
that `Outcome::scores_with_sigma` never had and whose own docs contradict;
rejection happens at ingestion as `InvalidParameter`. `event.rs` described
`add_events_with_prior` as replaced when it is still the ingestion
chokepoint. `factors.rs` advertised `Game::custom` without noting it is
`#[doc(hidden)]`. Internal T2/T4 milestone labels are dropped from public
items; the ones in the private `time_slice` module are left alone.

Closes #35

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014b6wy2q8rnFK8U8GPJVQNU

src/history.rs

@@ -554,13 +554,13 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
 
     /// 2-team win probability: returns `[P(team0 wins), P(team1 wins)]`.
     ///
-    /// N-team support lands in T4.
+    /// Only two teams are supported.
     ///
     /// # Panics
     ///
     /// Panics if `teams.len() != 2`.
     pub fn predict_outcome(&self, teams: &[&[&K]]) -> Vec<f64> {
-        assert_eq!(teams.len(), 2, "predict_outcome T2: 2 teams only");
+        assert_eq!(teams.len(), 2, "predict_outcome supports exactly 2 teams");
         let gather = |team: &[&K]| -> Gaussian {
             team.iter()
                 .filter_map(|k| self.keys.get(*k))