chore: draw the toolbar icons in-tree with make icons (closes #378)

script/lib/icons.js draws the mark from geometry read back out of the
committed PNGs, since the coordinates were never recorded, and writes each
PNG with node's own zlib. `make icons` runs it and leaves alone a file that
already holds the drawn image.

tests/icons.test.js requires each committed file to hold exactly that
image: the same IHDR and every pixel. The compressed bytes are not
compared, because node's bundled zlib does not compress as the stock zlib
that made the committed files did; the decision is recorded on the issue.

build.js copies the manifests from MANIFEST_SOURCES instead of naming the
two paths a second time.

Model: opus-5-5

README.md

@@ -317,6 +317,11 @@ The Makefile shims to those. It also carries a few targets that have no
 - `make build-debug` — the same build with `AUTISTMASK_DEBUG=1`, verified as a
   debug build, and keeping its `dist/` on failure (see
   [Debug Builds](#debug-builds))
+- `make icons` — redraw the toolbar icons in `icons/` from
+  `script/lib/icons.js`, at every size `manifest/chrome.json` declares. A file
+  that already holds the drawn image, the same IHDR and every pixel, is left
+  untouched. Commit the files with the drawing: `make check` fails while their
+  image differs from what it draws
 - `make clean` — remove `dist/` and `release/`
 - `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/`