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;