[test] test_balanced_matmul is flaky

[brnorris03]

test/python/pipe/test_mcast_matmul.py::test_balanced_matmul is flaky — passes and fails on the same commit.

Failing run: https://github.com/tenstorrent/tt-lang/actions/runs/24488320696/job/71567965917

FAILED test/python/pipe/test_mcast_matmul.py::test_balanced_matmul
AssertionError: PCC 0.000000 is below threshold 0.99.
Golden: mean=0.0002, std=0.0369.
Actual: mean=-138536127054300068350091329536.0000, std=22256473356137552083991291142078464.0000.

Passing run (same commit 9ab893e): https://github.com/tenstorrent/tt-lang/actions/runs/24485821826/job/71560420224

The garbage output (mean ~ -1.4e29) suggests uninitialized memory or a race condition in the multicast data path.