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/history.rs

@@ -261,6 +261,11 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
             let old = self.time_slices[j].posteriors();
 
             self.time_slices[j].new_backward_info(&self.agents);
+            self.observer.on_slice_processed(
+                &self.time_slices[j].time,
+                j,
+                self.time_slices[j].events.len(),
+            );
 
             let new = self.time_slices[j].posteriors();
 
@@ -280,6 +285,11 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
             let old = self.time_slices[j].posteriors();
 
             self.time_slices[j].new_forward_info(&self.agents);
+            self.observer.on_slice_processed(
+                &self.time_slices[j].time,
+                j,
+                self.time_slices[j].events.len(),
+            );
 
             let new = self.time_slices[j].posteriors();
 
@@ -292,6 +302,11 @@ impl<T: Time, D: Drift<T>, O: Observer<T>, K: Eq + Hash + Clone> History<T, D, O
             let old = self.time_slices[0].posteriors();
 
             self.time_slices[0].iteration(0, &self.agents);
+            self.observer.on_slice_processed(
+                &self.time_slices[0].time,
+                0,
+                self.time_slices[0].events.len(),
+            );
 
             let new = self.time_slices[0].posteriors();