Broaden FitnessObserver blanket impl to accept FnMut closures

[Copilot]

The blanket FitnessObserver impl was bounded on Fn, which excluded FnMut closures — a common pattern for observers that accumulate state (counters, logs, running stats, etc.), forcing unnecessary use of RefCell or similar wrappers.

Change

• Relaxed the bound in the blanket FitnessObserver<G> impl from Fn(&[(G, f32)]) to FnMut(&[(G, f32)])

This is a safe widening: every Fn is also FnMut, so no existing code breaks. The observe method already takes &mut self, making FnMut the natural bound.

// Now works without RefCell:
let mut count = 0usize;
let observer = |fitnesses: &[(MyGenome, f32)]| {
    count += 1;
    println!("Generation {count}: {} survivors", fitnesses.len());
};
simulator.run_with_observer(observer);

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