Fix Poisson PMF denominator for integer observations

[OldCrow]

Summary

• Replace factorial(k) with tgamma((double)k + 1.0) in poisson_pdf
• Avoid small-k inaccuracies from the Stirling-based factorial path
• Keep PMF computation numerically stable for integer observation counts

Validation

• Rebuilt and reinstalled StochHMM with this change
• Reran downstream libhmm vs StochHMM distribution suite
• Poisson nonfinite/null diff rows dropped from 9 to 0 in the benchmark artifacts

Co-Authored-By: Oz oz-agent@warp.dev