fix: make dev rebuilds dist/ on source changes (closes #332)

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

Makefile

@@ -107,6 +107,14 @@ vendor-blocklist:
 clean:
 	@rm -rf dist/ release/
 
+# Run the build make build runs, without the checks that follow it, then run it
+# again after every change to a file under src/, manifest/ or icons/, until
+# interrupted. A failed build is reported, may leave dist/ partly written, and
+# watching carries on. It writes no build receipt, so nothing can verify what it
+# leaves in dist/: anything handed on comes from make build. A release build
+# unless AUTISTMASK_DEBUG=1 is exported. A change anywhere else, package.json
+# and build.js included, starts no build, and a directory created while it runs
+# is not watched; restart it after either.
 dev:
 	@echo "Building in watch mode..."
 	@yarn run build --watch 2>&1