make dev passed --watch to a build.js that read no arguments, so it
built once and exited. build.js --watch now builds, then builds again
after every change to a file under src/, manifest/ or icons/, until
interrupted; any other argument fails. Each directory gets its own
watcher, because Node's recursive watch on Linux loses a file that an
editor saves by renaming a new copy over it. A watch build writes no
build receipt and cannot be verified; README.md and the Makefile say
so and point to make build.
Model: opus-5-5
