Muon pair distributions

- Process e+ e- -> mu+ mu-
- sqrt(s) = 240GeV
- no ISR, no FSR
- cross sections: excellent agreement between MADRAPH and Sherpa, KKMC off
- differential distribution (costheta): good agreement between MADGRAPH and KKMC, Sherpa off

| Generator | Xsection |
|---|---|
| MADGRAPH | 1.93128 |
| SHERPA | 1.93298|
| KKMC | 1.79665 |

[Muon240000antimuoncostheta1.pdf](https://github.com/user-attachments/files/30037179/Muon240000antimuoncostheta1.pdf)
[Muon240000muoncostheta0.pdf](https://github.com/user-attachments/files/30037178/Muon240000muoncostheta0.pdf)

Start by reproducing the e+ e- -> mu+ mu- comparison at sqrt(s) = 240GeV with ISR and FSR disabled, then inspect the MADGRAPH, Sherpa, and KKMC generator configuration and output for the reported cross-section and costheta discrepancies. Done would require identifying and correcting the relevant configuration or documenting why the generators disagree, but the issue names no files, tests, or acceptance criteria.

Written by the indexing model from the issue text.