Add multi-window service containers

[botoddly]

Implements the first multi-window delivery discussed in #382.

Summary

• add explicit-window application roots and per-window child service containers
• publish window sessions into root-owned rendering and SDL event coordination
• keep render managers, render phases, keyboard, mouse, and text input window-local
• split stable logical Window state from native ActivationWindow state
• make GraphicsPipelineBuilder transient so inherited resolution uses the requesting window container
• preserve the current implicit single-window composition model
• migrate the multi-window tutorial to sibling containers sharing root-owned GPU data
• keep StageManager application-scoped

Window reactivation and secondary-window Pencuil composition remain follow-up work.

Verification

• dotnet build GameKit.slnx --no-restore
• dotnet test GameKit.slnx --no-restore
• dotnet build tutorials/GameKit.Tutorials.MultiWindow/GameKit.Tutorials.MultiWindow.csproj --no-restore
• git diff --check

Graphical SDL execution was intentionally not added to the automated suite.

[stanoddly]

I feel like the name is strange, it's not schedule per se. It's just executing them in certain order, one by one. It's somehow a render manager of render managers.

Also, Publish and Unpublish sound really odd.

[stanoddly]

Forcing a non nullable item to be null? Not good.

[stanoddly]

Looks fishy.

[stanoddly]

Why not using?