Graphics: add mesh and task shader stage flags only when mesh shading is enabled #238

Current State

The global descriptor-set layout always includes MeshBitExt | TaskBitExt stage flags (RendererManager.cs:193-197, and the editor at SceneViewerService.cs:343), which is invalid on devices without VK_EXT_mesh_shader, including most Android GPUs.

Tasks

• Compute the stage flags from the device's mesh-shader support at both sites
• Add a regression test

Acceptance Criteria

• Validation-clean with mesh shading unsupported or disabled
• The test fails without the fix

Related

• Android plan task A5a · seniority Jr · mergeable now (.NET 10)
• Platform roadmap: Platform roadmap research: Steam Deck, Android, iOS, Web #100