Installing system update bricked an unrooted LG K7
I have solved my own problem, so am posting the resolution here for the benefit of future searchers who might be encountering the same or similar situation.
Most online guides to un-bricking a phone are written with the assumption that the phone in question is rooted, and the recommended solutions all proceed from that assumption.  My phone was not rooted, and most of the suggestions -- putting the phone into download mode, flashing a new ROM, etc. -- were beyond my skill-set (basically a complete novice).
However, one online guide (specifically, this one) included a piece of information that I didn't find elsewhere: namely,

Unbrick using LG Bridge (Mac &amp; Windows)
(If there is no KDZ available for your model or LG UP failed, try this)
(Official way to unbrick. If it fails, blame LG)

Download LG Bridge from here.
Install LG Bridge
Open LG Bridge
Connect device to computer and let LG Bridge install drivers
Go to the Software update tab
Click &quot;Update error recovery&quot; on bottom right
LG Bridge will download and flash the KDZ for you.


This turned out to be exactly what I needed.