l0qp1n5qr: Give the disabled sparkle button a solid readable face

tests/buttons.test.ts

@@ -27,6 +27,18 @@ function rule(selector: string): string {
   return m[1];
 }
 
+describe("SparkleButton disabled", () => {
+  it("keeps full opacity and a solid readable face", () => {
+    const body = rule(".btn-spark:disabled");
+    expect(body).toMatch(/opacity:\s*1\b/);
+    expect(body).toContain("var(--tile-3)");
+    expect(body).toContain("var(--ink-2)");
+    expect(body).toContain("not-allowed");
+    expect(readFileSync(path.resolve(__dirname, "../src/styles/legacy.css"), "utf8")).not.toMatch(/button:disabled\s*\{[^}]*opacity/);
+    expect(html(createElement(SparkleButton, { label: "Sign in", disabled: true }))).toContain("disabled");
+  });
+});
+
 describe("FlipButton states", () => {
   it("ready: POST plus send icon, flips to PUBLISH with the icon", () => {
     expect(flipLabels("ready")).toEqual({ front: "Post", back: "Publish", icon: true });