Scope cached method results to receiver and function identity

[SkBlaz]

Cached methods on different receivers currently share results when their method names and arguments match. For example, two networks returning degrees 3 and 8 both return 3 when using the same CacheManager. Distinct decorated functions with the same name also collide.

Scope each cache key to the decorated function and receiver identity in addition to the existing argument digest. Identity keys support equal and unhashable receivers and retain the receiver until its entry is evicted or cleared, preventing object-ID reuse. Named cache sizes and invalidation behavior are preserved. Document the receiver lifetime in AGENTS.md.

Validation:

• All 7 new regression cases fail against unchanged master.
• All 33 regression and existing lazy-evaluation tests pass on Python 3.12 and Python 3.9.
• Focused Ruff F/B checks and git diff --check pass.