DTLZ2: close remaining ~1.6× median IGD gap vs pymoo (15-seed)

[jkbennitt]

See docs/WILCOXON-RESULTS.md. C# median IGD ~0.0045 vs pymoo ~0.0028 (n=15). Convergence/diversity already same order after ASF fix; remaining candidates: niching batch selection order, SBX edge cases, eval accounting. Not a blocker for v0.1 announce.