[cpp] Immanence logic cleanup

[MowFord]

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 understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• [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?

Apologies for the PR re-creation that will be happening, but any of my older PRs that need changes will have to be re-made as I've lost access to the source branches.

I will try to do my best to re-create the comments to avoid extra work on LSB's side.

(I won't be putting this disclaimer on every new PR, just a link to the old)

https://github.com/LandSandBoat/server/pull/5525

Steps to test these changes

Good call on extending the duration, as the skillchain tier does affect the remaining duration of the skillchain effect.

Same testing behavior is useful: add prints to effects/skillchain.lua to get the explicit start/stop of the effect (Note that the effect isn't removed when a skillchain is created, so this below screenshot is the logs from the above skillchains)

and helixes