ci: run all examples including slow ones

[corneliusroemer]

Summary

After fixing the critical memory bug, all 27 examples now complete successfully. This PR enables testing of all examples in CI, including the previously "slow" examples that were timing out.

Changes

• Remove skip logic for slow examples (benefits_of_sex, genealogies_with_selection, measuring_fixation_probabilities, multi_sample_adaptive, mutation_selection_balance_highd, neutral_LD_highd, speed_highd, speed_lowd)
• Run all *.py examples serially in CI
• Simplify test commands (no more case statements)

Why This Works Now

Previously, examples like decay_of_LD_highd.py were failing after 5 generations with >10GB memory usage due to the integer overflow bug in provide_at_least(). With the fix in the base branch (#fix-makefile):

• Before: Try to allocate 2 billion clones → 100GB+ memory → timeout/kill
• After: Allocate only what's needed → <1GB memory → completes in seconds

Testing

All 27 examples pass locally:

cd examples && for f in *.py; do MPLBACKEND=Agg python3 "$f" || exit 1; done

✅ All examples complete successfully under 10GB memory limit

Base Branch

This PR is based on fix-makefile which contains:

• Integer overflow fix in provide_at_least()
• Memory leak fixes in free_mem()
• Python 3 compatibility fixes
• CI improvements (serial execution, native-only builds)