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

README.md

@@ -306,7 +306,15 @@ The Makefile shims to those. It also carries a few targets that have no
   debug build, and keeping its `dist/` on failure (see
   [Debug Builds](#debug-builds))
 - `make clean` — remove `dist/` and `release/`
-- `make dev` — build in watch mode
+- `make dev` — 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/` (see
+  [Build Receipts](#build-receipts)): 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
 
 ## End-to-End Tests