[core] Add plumbing for TH Proc System

[zach2good]

I affirm:

• [x] I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• [x] I have read and understood the Contributing Guide and the Code of Conduct.
• [x] I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Steps to test these changes

[TeoTwawki]

suggested pr title amendment: TH Proc System so people don't think this is "the one" that moves us to the retail chart for drop rates.

[zach2good]

I don't know enough about THF (despite having an i119 THF on retail) to do justice to the math on this one. WTB someone who knows what they're doing to help me! The code is easy, the logic is hard.