Expose Anderson acceleration stats in Python info

[bodono]

Summary

• update scs_source to the landed cvxgrp/scs master commit containing the shared AaStats plumbing from Expose Anderson acceleration stats in solve info cvxgrp/scs#386
• expose detailed AA diagnostics as sol["info"]["aa_stats"]
• add Python coverage for aa_stats keys and disabled-acceleration defaults
• mention aa_stats in the README example

Tests

• /tmp/scs-python-aa-venv/bin/python -m pip uninstall -y scs
• /tmp/scs-python-aa-venv/bin/python -m pip install .
• /tmp/scs-python-aa-venv/bin/python -m pytest -q test/test_scs_coverage.py -k "aa_stats or info_dict_has_expected_keys or all_info_keys_present or accel_steps_nonnegative"
• /tmp/scs-python-aa-venv/bin/python -m pytest -q test/test_solve_random_cone_prob_accelerate.py::test_solve_infeasible
• cmake -S scs_source -B /tmp/scs-aa-cmake -DBUILD_TESTING=ON -DUSE_LAPACK=ON
• cmake --build /tmp/scs-aa-cmake --parallel 4
• ctest --test-dir /tmp/scs-aa-cmake --output-on-failure