>One day the speed of light might be measured with a ruler and a clock. The next day, if the same measurement gave a different answer, no one could tell if the speed of light changed, the ruler length changed, or the clock ticking changed.

Not true.

The ruler and clock can change, but the speed of light ( or of every particle without mass) cannot.

The Relativity theory is founded on this axiom:
"the speed of light on the space" is a spacetime property.