refactor(RewardStreamerMP): keep track of `maxMP` instead of `potentialMP`

[0x-r4bbit]

This commit changes the mechanics to ensure there are no more MP generated that what's allowed as per max limiting.

Previously we've kept track of potentialMP which would decrease as more MP are generated.

This made verifying certain rules on certora hard and/or impossible. So we decided to track maxMP instead, which only decreases when users unstake.

This commit also introduces a rule that ensures any accounts MP never exceed their max mp.

Closes #44

Checklist

Ensure you completed all of the steps below before submitting your pull request:

• [ ] Added natspec comments?
• [x] Ran pnpm adorno?
• [x] Ran pnpm verify?

[0x-r4bbit]

Pinging @gravityblast