Epic: extract Gaya into its own repository #80

Goal

Gaya is the platform product, Turian is one set of plugins on it. Move Gaya to its own repository (MASS4ORG/Gaya) with its
own releases, once the seam is enforced and packaged.

Tasks

• Windows: verify and harden the Studio and CLI runtime #75 Enforce the Gaya → Turian boundary in CI
• macOS: MoltenVK runtime, Apple Silicon builds and Studio verification #76 Publish Gaya.Sdk and Gaya.Host to NuGet and consume them from Turian
• Gaya: enforce the Gaya → Turian dependency boundary in CI #77 Plugin discovery, manifest and distribution
• Move Gaya/ and the Gaya tests (Turian.Tests/Gaya) to the new repository with their own CI, release and docs
• Decide where the Gaya workbench issues live (Refactor: Move multi-DLL search logic to Editor.Core #15, In-game UI: keyboard and gamepad navigation #71) and transfer them
• Turian's Studio becomes a thin host: Gaya packages plus the GameStudio plugin

Acceptance Criteria

• Turian builds and releases with Gaya consumed only as packages.
• Gaya builds, tests and releases on its own.