feat: add OpenFDD Sandbox PWA (verbose reader + composer), sw.js, manifest.json, Try It links

Author: Corey B

site/index.html

@@ -126,6 +126,7 @@ <h2>The End of the Static PDF</h2>
                 Our architecture provides documents that are natively human-readable, cryptographically verifiable, and locally persistent without the need for complex infrastructure.
             </p>
             <a href="https://github.com/Spuds0588/open-fdd" class="button">Read the Specification on GitHub</a>
+            <a href="./sandbox.html" class="button" style="background:#38bdf8; color:#000; border-color:#38bdf8; margin-left:12px;">▶ Try the Live Sandbox</a>
         </section>
 
         <hr>

site/manifest.json

@@ -0,0 +1,16 @@
+{
+  "name": "OpenFDD Sandbox",
+  "short_name": "FDD Sandbox",
+  "description": "Live .fdd Reader, Verifier, and Composer. Zero dependencies — runs entirely in your browser.",
+  "start_url": "./sandbox.html",
+  "display": "standalone",
+  "background_color": "#0f172a",
+  "theme_color": "#0f172a",
+  "icons": [
+    {
+      "src": "https://img.shields.io/badge/FDD-Sandbox-38bdf8?style=flat",
+      "type": "image/svg+xml",
+      "sizes": "any"
+    }
+  ]
+}