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

build.js

@@ -588,15 +588,10 @@ async function build() {
         copyIcons(distDir);
     }
 
-    // copy manifests
-    copyEmitted(
-        path.join(__dirname, "manifest", "chrome.json"),
-        path.join(DIST_CHROME, "manifest.json"),
-    );
-    copyEmitted(
-        path.join(__dirname, "manifest", "firefox.json"),
-        path.join(DIST_FIREFOX, "manifest.json"),
-    );
+    // copy manifests, the same files copyIcons() read
+    for (const [distDir, manifestPath] of MANIFEST_SOURCES) {
+        copyEmitted(manifestPath, path.join(distDir, "manifest.json"));
+    }
 
     assertForbiddenTableCovered(forbiddenRecord);