feat: PartialEq on the config types, and pin the public trait impls

`Rating` already derived `PartialEq`, but that derive is only reachable
through `D: PartialEq` — and `ConstantDrift`, the crate's own only
`Drift` impl, did not satisfy it. So the derive was there and unusable.
Found by writing the comparison from a consumer's position rather than
reading the derive list.

`ConstantDrift`, `ConvergenceOptions` and `GameOptions` now derive
`PartialEq`. All three are pure configuration; comparing two is the
natural thing to want and nothing about them makes equality ambiguous.

`tests/trait_impls.rs` pins the surface, written the way the failure was
reported: a consumer struct that *holds* a `History` and derives
`Debug`. It also asserts `History`'s `Debug` summarises rather than
dumping its skill stores, so a future derive cannot quietly replace the
hand-written impl.

`Clone` on `History` stays off. It is a decision, not an omission: a
history owns every slice's skill store and arena, so cloning one is
proportional to the whole fit, and no consumer has wanted it.

Closes #76.

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

src/convergence.rs

@@ -4,7 +4,7 @@ use std::time::Duration;
 
 use smallvec::SmallVec;
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub struct ConvergenceOptions {
     pub max_iter: usize,
     pub epsilon: f64,

src/drift.rs

@@ -43,7 +43,7 @@ pub trait Drift<T: Time>: Copy + Debug + Send + Sync {
 /// A non-finite gamma is caught a second time regardless:
 /// `History::converge` validates the drift variance each competitor actually
 /// accumulates, which also covers a custom [`Drift`] implementation.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub struct ConstantDrift(f64);
 
 impl ConstantDrift {

src/game.rs

@@ -68,7 +68,7 @@ impl DiffFactor {
 /// `p_draw` and `convergence` apply to ranked outcomes (`Game::ranked`).
 /// `score_sigma` applies only to scored outcomes (`Game::scored`); it controls
 /// how much the engine trusts the observed score margin (smaller σ = more trust).
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub struct GameOptions {
     pub p_draw: f64,
     pub score_sigma: f64,