keysDifferingOnlyInANonAsciiLetterAddressDistinctEntries

Two keys differing only in a non-ASCII letter.

This is the pair the two legacy sanitisers disagreed on, which is why the encoding is now one shared thing rather than two that must agree by inspection: [^a-zA-Z0-9_-] folded мир and миг both to ___, while isLetterOrDigit() — true for Cyrillic — kept them apart. A key that survived on Apple collided on JVM, and each backend's own tests were green.

Keys derived from anything a person typed reach this.