osrelease: defer README example via let detect ()
The block ran at top level so mdx executed [Osrelease.OS.v ()] /
[Distro.v ()] and printed machine-specific output ("Running on macos
/ arm64") which then failed the diff. Wrap as [detect () = ...],
switch [Format.printf]/[Format.eprintf] to [Fmt.pr]/[Fmt.epr], add
[fmt] to the mdx libraries.