Compare what the two paths produced

### Task Summary

Two frames agree when they hold the same rows in the same order with the same values, and the comparison should say which of those failed rather than only that they differ. A float is compared within a tolerance, since the two paths reach the same number by different arithmetic.

A visualization has no frame to compare. What it emits is a figure, and the meaning of a figure is in the numbers behind it, so the figures are compared number by number; a plot that renders as an image is compared as the HTML it emits instead.

Step 6 of 27 in #8325. It stands on its own: it touches no code from the export itself.