I finally solved the mystery!
The culprit: BBEdit!
I used BBEdit to create a new .htaccess file cuz I didn't know nuthin bout no pico or vi or whatever. I had been using BBEdit to change httpd.conf and such and it worked just fine, but unbeknownst to me it had created .htaccess with MAC-style line-breaks, not UNIX-style ones.
I guess cuz httpd.conf was a pre-made file it already had Unix line breaks and BBEdit kept it that way, but when I created a NEW file, it decided that Mac line breaks would be ever so much nicer. I only figured this out after poking around in BBEdit's "save as" options. Once I changed the "save as" to Unix line breaks, password authentication began working like a charm.
I guess the Mac-Unix convergence ain't exactly all converged and such.