Simplify recursion in ObjCClass method loading

[samschott]

This PR simplifies and documents the recursive approach to method loading for an ObjCClass:

1. Document for instance_method_ptrs, instance_methods, and partial_methods what they contain and when they are populated.
2. Accommodate changes to class hierarchy during init.
3. Limit recursive method loading to ObjCClass._load_methods only.

I still want to test this with toga to see if there are any edge cases which are not covered by Rubicon's test suite.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[samschott]

It looks like there are some issues with getattr returning an ObjCBoundMethod instead of performing attribute access in some cases: https://github.com/beeware/toga/actions/runs/12198109536/job/34029274391?pr=2978