feat(api): add Observer trait and NullObserver default

Observer replaces verbose: bool with structured progress callbacks:
on_iteration_end, on_batch_processed, on_converged — all no-op
default impls so users override only what they need. NullObserver
is a ZST default.

Send + Sync bounds deferred to T3 (Rayon support).

Fully additive — wired into History::converge in Task 12.

Part of T2 of docs/superpowers/specs/2026-04-23-trueskill-engine-redesign-design.md.

src/lib.rs

@@ -19,6 +19,7 @@ pub mod gaussian;
 mod history;
 mod key_table;
 mod matrix;
+mod observer;
 mod outcome;
 mod rating;
 pub(crate) mod schedule;
@@ -33,6 +34,7 @@ pub use gaussian::Gaussian;
 pub use history::History;
 pub use key_table::KeyTable;
 use matrix::Matrix;
+pub use observer::{NullObserver, Observer};
 pub use outcome::Outcome;
 pub use rating::Rating;
 pub use schedule::ScheduleReport;

src/observer.rs

@@ -0,0 +1,48 @@
+//! Observer trait for progress reporting during convergence.
+//!
+//! Replaces the old `verbose: bool` + `println!` path. Callers wire in any
+//! observer that implements the trait; default methods are no-ops so users
+//! override only what they need.
+
+use crate::time::Time;
+
+/// Receives progress callbacks during `History::converge`.
+///
+/// All methods have default no-op implementations; implement only what's
+/// interesting. Send/Sync is NOT required in T2 (added in T3 along with
+/// Rayon support).
+pub trait Observer<T: Time> {
+    /// Called after each convergence iteration across the whole history.
+    fn on_iteration_end(&self, _iter: usize, _max_step: (f64, f64)) {}
+
+    /// Called after each time slice is processed within an iteration.
+    fn on_batch_processed(&self, _time: &T, _slice_idx: usize, _n_events: usize) {}
+
+    /// Called once when convergence completes (or max iters is reached).
+    fn on_converged(&self, _iters: usize, _final_step: (f64, f64), _converged: bool) {}
+}
+
+/// ZST no-op observer; the default when none is configured.
+#[derive(Copy, Clone, Debug, Default)]
+pub struct NullObserver;
+
+impl<T: Time> Observer<T> for NullObserver {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn null_observer_compiles_for_i64() {
+        let o = NullObserver;
+        <NullObserver as Observer<i64>>::on_iteration_end(&o, 1, (0.0, 0.0));
+        <NullObserver as Observer<i64>>::on_converged(&o, 5, (1e-6, 1e-6), true);
+    }
+
+    #[test]
+    fn null_observer_compiles_for_untimed() {
+        use crate::Untimed;
+        let o = NullObserver;
+        <NullObserver as Observer<Untimed>>::on_iteration_end(&o, 1, (0.0, 0.0));
+    }
+}