⚙️ 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>

go.mod

@@ -1,3 +1,3 @@
-module github.com/nunocoracao/blowfish/v2
+module github.com/nunocoracao/blowfish/v3
 
 go 1.16