Preserve depletion Sidak correction behavior

[wes-lewis]

Summary

This preserves edge-case Sidak correction behavior in the depletion test with the behavior in the original locat_condensed.py.

In locat-0.1, logsidak_from_logp() returned the unadjusted logp_min whenever logp_min <= 0, which is the ordinary case for log-p values. That effectively skipped the Sidak correction in the depletion scan.

This change restores the original behavior from locat_condensed.py:

• only skip the correction when there is effectively a single test (m_eff <= 1)
• otherwise apply the Sidak correction in log-space

Why

On local comparison runs, this minimal helper fix was sufficient to bring the locat-0.1 depletion behavior much closer to the original locat_condensed results in the null-calibration simulation, without changing the rest of the locat-0.1 scan path.

Scope

This PR only changes logsidak_from_logp() in locat/locat.py.

[fra-pcmgf]

I updated the test result. I'm merging the code to main and release a new version