Easiest way to build a one-off iPhone "App" (probably JS on a local web page)?
I asked this as a genuine question, but I'm now answering it myself since I found a technique that works for me and it might help others.

I wrote a basic old-school HTML + Javascript page that did the required calculation, then uploaded it to my web server along with the manifest file described here. Having visited the page once, my phone has cached it and can now re-open it (including re-running the Javascript to give the new result) even without Internet access.