I tried a couple more things.  I re-defined _ns_http_puts with the original definition and everything worked fine.  This got me curious so I ran [info body _ns_http_puts] (in a new interpreter where I hadn’t redefined the proc) and of course I got “ƒÄ,¶——“.

A workaround is obvious, but the mystery remains as to how the proc got redefined with the gobbledygook in the first place.  It might not be ns_sockselect after all, though I was able to narrow it down to that in the 4.0.10 case.  It must happen before my custom libraries are processed or else my redefinition wouldn’t fix the problem.