Allow FitnessObserver::observe and Repopulator::repopulate to take &mut self

[Copilot]

FitnessObserver::observe and Repopulator::repopulate previously took &self, forcing implementations that need to track state to use interior mutability (Cell, Mutex, etc.). Since these are not shared across threads, &mut self is sufficient and more ergonomic.

Changes

• FitnessObserver::observe: &self → &mut self
• Eliminator::eliminate: &self → &mut self (required to propagate mutability through FitnessEliminator::eliminate → self.observer.observe(...))
• Repopulator::repopulate: &self → &mut self
• All implementations updated: (), LayeredObserver, FitnessEliminator, KnockoutEliminator, MitosisRepopulator, CrossoverRepopulator, SpeciatedCrossoverRepopulator
• Tests updated: CountingObserver impl and mut layered bindings

Example

Observers and repopulators can now hold mutable state directly without interior mutability:

struct LoggingObserver {
    count: usize,
    history: Vec<f32>,
}

impl<G> FitnessObserver<G> for LoggingObserver {
    fn observe(&mut self, fitnesses: &[(G, f32)]) {
        self.count += 1;
        if let Some((_, best)) = fitnesses.first() {
            self.history.push(*best);
        }
    }
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Allow FitnessObserver to take a mutable reference</issue_title>
<issue_description>It's not being sent across threads or anything, so there's really no complications or anything preventing us from passing a mutable reference. Makes maintaining observers easier since virtually every case needs to mutate some internal state.</issue_description>

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

• Fixes Allow FitnessObserver to take a mutable reference #138

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[HyperCodec]

@copilot do the same for repopulators for consistency.

[Copilot]

@copilot do the same for repopulators for consistency.

Done in 81b38a1. Repopulator::repopulate and all three implementations (MitosisRepopulator, CrossoverRepopulator, SpeciatedCrossoverRepopulator) now take &mut self.