Mutate first offspring genome instead of host genome

[chrxh]

Summary

Move genome mutation so it applies to the first offspring's own cloned genome instead of the host genome.

Previously mutateGenome cloned and mutated the host creature's genome in place (before it was cloned for the offspring). Now:

• processCell: the offspring creature is created (findOrCreateNewCreature) before mutateGenome, so the mutation targets the offspring.
• mutateGenome: mutates offspring->genome, guarded by currentOffspring == 0 && createsNewCreature(...); the host genome is left untouched. Once-guard via atomicExch(offspring->mutationState).
• findOrCreateNewCreature: the first offspring (currentOffspring == 0) gets its own genome copy (cloneGenome) so the mutation does not affect the host.

Result: only the first offspring is mutated, on its own genome copy; the parent keeps its original genome.

Testing

Not built/run here — the vcpkg submodule is not provisioned in this environment. Tests intentionally left unchanged.

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