Why I like awk
The fix turned out to be one line, after an hour of looking in the wrong place. Next time I will start with the boring solution and see if it is enough. I wish someone had told me this when I started. It is worth writing down the commands you only need twice a year.
I keep relearning this, which is exactly why it goes here. I ran into this again last week, so here is a short note for future me. The documentation was correct, I just did not read it carefully. Measuring before changing anything saved me from a pointless rewrite.
Most of the time the simplest tool is good enough. After cleaning things up, the whole thing runs noticeably faster. Plain text keeps winning for small personal projects.
Writing a tiny test first made the problem obvious. There is probably a better way, but this one is easy to remember.
Naming things clearly helped more than any clever trick.