Restrict SpeciatedPopulation::species to enforce non-empty species invariant

[Copilot]

SpeciatedPopulation had fully public fields, allowing external construction with empty inner Vecs and causing panics in round_robin/round_robin_enumerate (index/modulo by zero).

Changes

• speciation.rs — Make species field private (invariant: every inner Vec is non-empty). threshold stays pub. Add:
  • SpeciatedPopulation::new(threshold) — empty population constructor
  • fn species(&self) -> &[Vec<usize>] — read-only accessor
• eliminator.rs / repopulator.rs — Migrate all population.species field accesses to population.species()
• tests/speciation.rs — Replace struct literal construction with new() + insert_genome(); update all field accesses to the accessor

Before / After

// Before — could be constructed in an invalid state
let mut pop = SpeciatedPopulation {
    species: vec![vec![]],  // empty inner vec → panic in round_robin
    threshold: 0.5,
};

// After — only valid states reachable through the public API
let mut pop = SpeciatedPopulation::new(0.5);
pop.insert_genome(0, &genomes, &());  // always pushes vec![index]

let species: &[Vec<usize>] = pop.species();

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[HyperCodec]

could be better, but it's good enough (I doubt anyone will use this struct directly anyway)