⚡ Optimize SumTree recursion to iterative loops

[google-labs-jules]

Replaced recursive implementations of _propagate and _retrieve in per.py with iterative while loops. This avoids recursion overhead and potential stack overflow issues for very deep trees.
Benchmark results showed a ~4% performance improvement on 100k updates.
Added tests/test_per.py to ensure correctness of SumTree operations.

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PR created automatically by Jules for task 17481305502146844519 started by @dzaczek

[google-labs-jules]

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