Gaya: enforce the Gaya → Turian dependency boundary in CI #77

V2 sequencing (2026-10-01): Extend the boundary check to MASS4.Attributes/Guinevere generic forms, and ensure shipped runtime/simulation consumers cannot accidentally pull in Editor/Gaya.Host or a mandatory Vulkan/window backend. Keep Gaya.Packages usable independently.

Current State

Gaya/ (Gaya.Sdk, Gaya.Host) has no references to Turian today. Nothing keeps it that way while both live in one
repository, and one stray using Turian.… would turn the future extraction back into archaeology.

Tasks

• A test (or MSBuild check) that fails when any Gaya/** project references a Turian.* project, package or
  namespace
• Same check for Gaya tests: they must not need Turian types
• Run it in ci.yml

Acceptance Criteria

• Adding a Turian reference to any Gaya project fails CI with a clear message.