WG21-P0952R2 A New Specification For generate_canonical()

No feature-test macro is mentioned.

This technically isn't a Defect Report, but it's a behavioral change that resolves an LWG issue and avoids the correctness-damaging behavior of returning 1.0, so we want to implement this unconditionally and WILL accept a PR for this in advance of opening up the floodgates to arbitrary C++26 features at some point in the future.