docs: record the intermittent test_mainwindow failure as item 61

Two tests fail about one full run in twenty, and neither is caused by the card
list: a worktree at f72dba9, before any of that work, failed 3 of 12 runs, and
neither test was touched by it.

Recorded with what was measured and what was not. The cause is NOT established:
synthetic CPU load did not reproduce it, so the trigger is narrower than "the
machine is busy", and the entry says so rather than shipping a plausible guess
for the next reader to trust.

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