refactor!: close the remaining API gaps from #21

Three unrelated small defects, all requiring signature changes:

- `Game::one_v_one` hardcoded `GameOptions::default()`, so a 1v1 could
  never set `p_draw` or convergence options — and a drawn 1v1 was
  therefore unreachable through it, since the default `p_draw` is zero.
  It now takes `&GameOptions` like every other constructor.

- `Observer::on_batch_processed` was declared on the trait and never
  called from anywhere: implementors wired up a callback that could not
  fire. It is now called after each slice sweep, and renamed
  `on_slice_processed` to match the vocabulary the codebase adopted in
  T2 — the unit of work is a `TimeSlice`, not a batch. A slice is swept
  once travelling backward and once forward, so a multi-slice history
  fires it twice per slice per iteration; the doc comment says so.

- `pub mod factors` sat beside `pub(crate) mod factor`, two module paths
  differing by one character with only one of them importable. The
  public facade is now `graph`.

Tests cover each as a behaviour rather than a compile check: a drawn 1v1
succeeds only when p_draw is supplied, and the observer tests fail if
any callback stops firing.

BREAKING CHANGE: `Game::one_v_one` takes a fourth `&GameOptions`
argument; `Observer::on_batch_processed` is renamed
`on_slice_processed`; the `factors` module is renamed `graph`.

Closes #21

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

src/game.rs

@@ -532,18 +532,20 @@ impl<T: Time, D: Drift<T>> Game<'_, T, D> {
         ))
     }
 
+    /// Convenience wrapper over [`Game::ranked`] for two single-player teams.
+    ///
     /// # Errors
     ///
-    /// Delegates to [`Game::ranked`] with default options, so it returns the
-    /// same errors — in practice `WrongOutcomeKind` for a non-ranked outcome,
-    /// or `TieWithoutDrawProbability` for a draw, since the default `p_draw`
-    /// applies rather than one you chose.
+    /// Delegates to [`Game::ranked`], so it returns the same errors — in
+    /// practice `WrongOutcomeKind` for a non-ranked outcome, or
+    /// `TieWithoutDrawProbability` for a draw when `options.p_draw` is zero.
     pub fn one_v_one(
         a: &Rating<T, D>,
         b: &Rating<T, D>,
         outcome: crate::Outcome,
+        options: &GameOptions,
     ) -> Result<(Gaussian, Gaussian), crate::InferenceError> {
-        let game = Self::ranked(&[&[*a], &[*b]], outcome, &GameOptions::default())?;
+        let game = Self::ranked(&[&[*a], &[*b]], outcome, options)?;
         let post = game.posteriors();
         Ok((post[0][0], post[1][0]))
     }
@@ -563,11 +565,11 @@ impl<T: Time, D: Drift<T>> Game<'_, T, D> {
     }
 
     #[doc(hidden)]
-    pub fn custom<S: crate::factors::Schedule>(
-        factors: &mut [crate::factors::BuiltinFactor],
-        vars: &mut crate::factors::VarStore,
+    pub fn custom<S: crate::graph::Schedule>(
+        factors: &mut [crate::graph::BuiltinFactor],
+        vars: &mut crate::graph::VarStore,
         schedule: &S,
-    ) -> crate::factors::ScheduleReport {
+    ) -> crate::graph::ScheduleReport {
         schedule.run(factors, vars)
     }
 }