Thank you. I'm not sure why every search I tried yielded folks giving examples of using Rewrite rather than Redirect.
While I'm looking to redirect just a subdirectory, rather than everything, once I was put onto the track of Redirect I found that's do-able too:
where the subdirectory argument with the Redirect had to be not or .
Redirect permanent /subdir https://www.example.com/subdir
(I included the "permanent" option since it sends a different status code, 301; apparent the default is "temporary" code 302 -- I'm not sure in what contexts that is important but this redirection is designed to be permanent in nature so I figured it's better to be precise.)