Fix: Prevent inf values in clicks/impressions when CPC/CPM becomes non-positive

[takechanman1228]

Summary

• Fix simulate_media to floor non-positive CPC/CPM to a small positive percentile-based value (prevents divide-by-zero and inf in lifetime_clicks/impressions when additive noise drives cost <= 0).
• Add regression test covering the deterministic CPC noise scenario that previously produced inf clicks for click channels.

Issue #43

Solution

Modified _negative_replace() to handle CPC/CPM columns specially:

• CPC/CPM columns: Replace non-positive values with the 1st percentile of positive values (minimum 1e-6)
• Other columns: Unchanged behavior (replace with 0)

[takechanman1228]

Hi @RyanAugust , thanks for maintaining this project. Whenever you have time, could you please review this PR? Any feedback appreciated.