Add QoL trait impls to LayeredObserver

[Copilot]

LayeredObserver<G, A, B> lacked standard trait derivations, making it awkward to use in contexts requiring default construction, cloning, equality checks, or debug formatting.

Changes

• Default — constructs via A::default() and B::default()
• Clone — deep-clones both inner observers
• Debug — formats as LayeredObserver(a, b)
• PartialEq / Eq — delegates equality to both inner observers

All impls are conditional on A and B satisfying the respective bound — no constraints imposed on G.

Example

#[derive(Default, Clone, Debug, PartialEq)]
struct MyObserver;

impl<G> FitnessObserver<G> for MyObserver { /* ... */ }

// Now works:
let observer = MyObserver.layer(MyObserver);
let cloned = observer.clone();
let default: LayeredObserver<(), MyObserver, MyObserver> = LayeredObserver::default();
assert_eq!(observer, cloned);
println!("{:?}", default); // LayeredObserver(MyObserver, MyObserver)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add some extra traits to LayeredObserver</issue_title>
<issue_description>Can do something like this:

impl<G, A, B> Default for LayeredObserver<G, A, B>
where
    A: Default + FitnessObserver<G>,
    B: Default + FitnessObserver<G>,
{
    fn default() -> Self {
        Self(A::default(), B::default());
    }
}
```</issue_description>

<agent_instructions>Add whatever traits you feel would be convenient for QoL.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Add some extra traits to LayeredObserver #141

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[HyperCodec]

tbh I don't really see why tests are necessary for such a trivial change, but sure.