[lua] Implement high/low pdif rolls for melee

[WinterSolstice8]

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What does this pull request do?

Implements the 50/50 high/low pdif selector for super low pdif values based on source cited in the code and word from Jimmayus (and other math nerds)

Fixes #9783

Steps to test these changes

Have ultra low pdif (1 attack vs 20k+ def mob), sometimes hit for non-zero with at ( weapon damage minus fstr) * .5 *1.05 and half of the time for zero

22 base damage weapon: