Simply Screens 0.8.8

Fixes

• Removed obsolete empty Sponge Mixin configuration from the supported builds.
• Eliminated the startup warning about the missing Simply Screens refmap on production Forge 1.20.1 servers and clients.
• Removed the corresponding dead Fabric and Forge mixin registrations instead of packaging a placeholder refmap.

CI and release safety

• Added a production-jar assertion that rejects simplyscreens.mixins.json or a Simply Screens refmap if either is accidentally packaged again.
• Applied the assertion to pull-request live-join validation and release builds before artifacts are uploaded.

This is a packaging and CI maintenance release; no gameplay or screen behavior changes are intended.