fix: declare toolbar icons and ship real PNGs in both archives (closes #371)

Neither manifest declared an `icons` block, so Chrome and Firefox both drew the
generic puzzle piece for this extension. That is the first thing the owner sees
on every launch, and an unbranded placeholder is also how a user fails to tell a
real extension from a look-alike.

Both manifests now declare 16/32/48/128 as `icons/icon<size>.png`, and the four
PNGs live at `icons/` in the tree. build.js copies them into each browser
directory relative to it, so nothing points up and out the way `dist/styles.css`
does, and the receipt records them like every other emitted file.

The sizes copied come from the manifest that will ship next to them, not from a
second list in build.js: a size a manifest declares and `icons/` does not hold
fails the build with the path that is missing, rather than emitting a directory
whose manifest references nothing. script/lib/package.js already resolved `.png`
strings, so an icon that reached a manifest but not the archive fails
self-containment; tests/packaging.test.js now pins that case, since it was
covered only incidentally before.

tests/manifest.test.js asserts the declaration in both manifests, that both
declare the same set, and that each referenced file is a PNG whose IHDR states
the size the entry claims — a declaration alone would still permit a reference
to a file that is not there or is not an image.

The artwork is original, drawn from geometry rather than traced or downloaded: a
flat dark-navy rounded square (#101A2E) with a teal (#35E0C2) triangular "A" —
one outer triangle minus a triangular counter — rasterised with 8x8 supersampling
and encoded as RGBA PNG. One shape, two flat colours, which is what a 16px
toolbar slot can carry.

Verified: make check 56 suites / 1023 tests, make build and make package green,
both archives unpacked and the four icons confirmed inside each with bytes
identical to the tree, make test-e2e 55/55 and 5/5, make test-e2e-firefox 8/8 and
7/7 (the latter installs the packaged XPI). Removing icons/icon48.png fails
make build; making build.js skip one copy fails make package with "the chrome
archive would not be self-contained: it is told to load icons/icon48.png, which
is not in it".

tests/manifest.test.js

@@ -47,6 +47,12 @@ const fs = require("fs");
 const path = require("path");
 
 const MANIFEST_DIR = path.join(__dirname, "..", "manifest");
+const ROOT = path.join(__dirname, "..");
+
+// The sizes both stores and both toolbars ask for.
+const EXPECTED_ICON_SIZES = ["16", "32", "48", "128"];
+
+const PNG_SIGNATURE = Buffer.from("89504e470d0a1a0a", "hex");
 
 const EXPECTED_DIRECTIVES = {
     "default-src": ["'self'"],
@@ -115,6 +121,51 @@ function assertPolicy(policy) {
     }
 }
 
+// The declared icons, in both manifests.
+//
+// Without an "icons" block a browser draws a generic puzzle piece in the
+// toolbar for this extension, which is both the first thing the user sees and
+// how a real extension is told apart from a look-alike. Declaring one is not
+// enough on its own: an entry naming a file that is not in the tree ships a
+// reference to nothing, so the referenced bytes are read here and required to
+// be a PNG of the size the entry claims. build.js copies these into each
+// browser directory, relative to it, and script/lib/package.js then refuses to
+// build an archive that does not contain everything the manifest names.
+function assertIcons(target) {
+    const icons = readManifest(target).icons;
+    expect(Object.keys(icons).sort()).toEqual(EXPECTED_ICON_SIZES.sort());
+    for (const size of EXPECTED_ICON_SIZES) {
+        const ref = icons[size];
+        expect([size, ref]).toEqual([size, `icons/icon${size}.png`]);
+
+        const bytes = fs.readFileSync(path.join(ROOT, ref));
+        expect(bytes.subarray(0, 8)).toEqual(PNG_SIGNATURE);
+        // IHDR width and height, at fixed offsets right after the signature
+        // and the chunk header.
+        expect([ref, bytes.readUInt32BE(16), bytes.readUInt32BE(20)]).toEqual([
+            ref,
+            Number(size),
+            Number(size),
+        ]);
+    }
+}
+
+describe("declared icons", () => {
+    test("chrome declares real icons at every size", () => {
+        assertIcons("chrome");
+    });
+
+    test("firefox declares real icons at every size", () => {
+        assertIcons("firefox");
+    });
+
+    test("both targets declare the same icons", () => {
+        expect(readManifest("firefox").icons).toEqual(
+            readManifest("chrome").icons,
+        );
+    });
+});
+
 describe("shipped Content Security Policy", () => {
     // MV3 takes an object and applies extension_pages to the popup and the
     // background service worker, which is where libsodium runs.