docs: clean up & document game speed-related code

[Bertie690]

What are the changes the user will see?

N/A

Why am I making these changes?

The game speed code was not well documented, and also specified many lengthy types that could (and should) be inferred.

I was particularly motivated by mutateProperties, which had a parameter type of any and a biome ignore that brushed a larger issue aside.

What are the changes from a developer perspective?

Added documentation to various functions and methods used by the game speed code, removing type specifications where TS was able to infer them.

Altered obj inside mutateProperties to be object for slightly more safety, and mentioned the fact that the method would be within allowed bounds were it not defined inside the initGameSpeed function.
(For those curious, I just added oneline if statements until it started giving errors, since each one is worth 1 point without nesting.)

Screenshots/Videos

N/A

How to test the changes?

N/A

Checklist

• The PR content is correctly formatted:
  • I'm using beta as my base branch
  • The current branch is not named beta, main or the name of another long-lived feature branch
  • I have provided a clear explanation of the changes within the PR description
  • The PR title matches the Conventional Commits format (as described in CONTRIBUTING.md)
• The PR is self-contained and cannot be split into smaller PRs
• There is no overlap with another open PR
• The PR has been confirmed to work correctly:
  • I have tested the changes manually
  • The full automated test suite still passes (use pnpm test:silent to test locally)