docs(todo-format): warn against double-counting rarity in the bug matrix

Grading a bug's severity by how rare it is to enter the failure state double-counts the frequency axis, which already carries that rarity. It buries the compounding bugs, the ones where a rare trigger produces unbounded harm. Three rules added to the severity × frequency section: grade severity by the rate of harm once the failure state is entered, record the band, row, and arithmetic in the task body so a misgrade can be re-checked against the source, and disagree with a grade by fixing an input rather than overriding the letter.

A chime watchdog bug graded [#D] moved to [#C] an hour later, once the read was written down and re-checked.

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