⚙️ Prepare v3 release: module path, version window, 3.0.0

Bump the Hugo module path to /v3, adopt a rolling latest-4 Hugo
version window (bot keeps it sliding), document new opt-in params,
and set package version to 3.0.0.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "hugo-blowfish-theme",
-  "version": "2.106.0",
+  "version": "3.0.0",
   "description": "Blowfish theme for Hugo.",
   "scripts": {
     "postinstall": "vendor-copy",