feat(rune): Add reverse and forward mode AD for cummax() and cummin()

[kuku929]

This is a first draft implementation. Some functionality (e.g. scatter_add()) is still missing, but the main logic is in place.

Notes:

• The helper functions contain temporary comments meant to clarify the current logic. These will be rewritten before the final commit to match the contributing guidelines.
• I’ve already noticed a few indentation issues. If you spot more, please let me know — I’ll fix them all in a single formatting commit.
• There’s currently one test case included. I plan to add more later, but this matches the pattern in other functions for now.