Gaya: publish Gaya.Sdk and Gaya.Host as NuGet packages and consume them from Turian #78

Current State

Gaya.Plugin.GameStudio and Turian.Editor.Studio reference Gaya.Sdk / Gaya.Host as projects. Gaya has no
PackageId and no version of its own.

Tasks

• PackageId MASS4.Gaya.Sdk and MASS4.Gaya.Host, with Gaya versioned independently of Turian
• Pack and publish both from the release pipeline
• Turian consumes them as PackageReference, with a local-path toggle for development — the same pattern as
  GuinevereLocalPath in Turian.Engine.UI.csproj
• Semantic versioning policy for the SDK surface (what counts as a breaking change for plugins)

Acceptance Criteria

• Turian builds against the published packages with no Gaya project references.
• Switching to local Gaya sources is one property.

This is the change that makes the repository split mechanical.