[tenth-package0]

Summary

Fix the eviction order in SlidingWindowAccumulator. With a window size of 3, adding [1, 2, 3, 4] currently leaves [1, 2, 4] and returns a mean of 7 / 3, instead of retaining the latest three values and returning 3.

Keep the replacement index at zero while the buffer fills, then advance it only when replacing an entry. Use modular index advancement to make wraparound explicit. Reject non-positive capacities with a clear ValueError instead of failing later with an index error.

Add regression coverage for the first eviction, an empty window, invalid capacities, capacity one, partial windows, and repeated wraparound at several capacities. Register the test in the Python CMake test list.

Validation

• python -m open_spiel.python.utils.stats_test: all 8 tests pass on Python 3.12.14; the original implementation fails the eviction regression cases.
• ruff check open_spiel/python/utils/stats.py open_spiel/python/utils/stats_test.py: passed.
• pyink --check open_spiel/python/utils/stats_test.py: passed.
• git diff --check: passed.

The full C++/Python integration suite was not run locally.

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