fix: site-tokens - Pair the primary button text with the fill of the same buttons #4018

Found in scaffold end-to-end test run 2 (9 Nov 2026, finding N5).

Problem

On TSP, action/primary/background is primary-600 (#6a6d5b, the fill on 10 buttons), but action/primary/text is brand-600, taken from #9b6a22, the text on a different set of 112 buttons that have a #f7f3f1 fill. Text and fill come from different buttons, so they contrast at 1.01:1 (1.57:1 on hover). The report lists the failure, but the pairing is wrong in the first place.

Fix

Take action/*/text from the buttons whose fill is the chosen background. If those buttons give no readable colour, use the default and pick a passing step.

Done when

On the TSP crawl, the primary action text and background come from the same buttons and pass 4.5:1, or the report explains why not.