Handle unregistered template changes gracefully

[oiahoon]

Summary

• keep loaded editors editable by returning to the existing fallback display when their template name is unregistered
• resume highlighting when the requested template is registered and release stale waiting-queue entries
• cover missing, stale, recovered, and subsequent registered-template transitions

Fixes #227

Testing

• Prism browser test page: all tests passed
• Highlight.js browser test page: all tests passed
• node --check code-input.js
• node --check tests/tester.js
• git diff --check

[WebCoder49]

Thanks for testing! Unfortunately I won't be able to review this until after EMF2026 and then work, next Monday at the very earliest. I hope that's fine since you don't use code-input.js.