Add render loop frame provider

[Aaronontheweb]

Summary

• add an application-owned R3 frame provider and loop dispatch helpers
• marshal timer, async stream, and reactive layout demo updates through the render loop
• document render-loop threading and update tutorials/component docs with regenerated demo GIFs

Validation

• dotnet build Termina.slnx -c Release --no-restore
• dotnet test tests/Termina.Tests/Termina.Tests.csproj -c Release --no-build
• npm run build
• vhs screenshots/tapes/streaming-chat.tape
• vhs screenshots/tapes/grid-dashboard.tape
• vhs screenshots/tapes/gallery-animations.tape