Bricks: platform allow-list and per-platform NuGet in brick manifests #248

Current State

Brick manifests (Gaya/Gaya.Packages/PackageManifest.cs, docs/bricks/package.schema.json) have one nuget map and client/server/editor flags, but no platform constraint. The UI brick lists desktop-only SkiaSharp natives and the vulkan brick lists GLFW; BuildProfiles.ShippingBricks filters only Client vs Server.

Tasks

• Add a platforms allow-list (absent = any platform)
• Add per-platform NuGet entries (decide naming)
• ShippingBricks and CsProjectGenerator.AddBrickPackages honor the profile's platform
• Export fails early naming the unsupported brick
• Update the schema and brick docs

Acceptance Criteria

• Tests for platform filtering and the failure message
• Desktop export unchanged

Related

• Android plan task A12 · seniority Mid · mergeable now (.NET 10)
• Depends on: Build: IBuildPlatform owns the platform-specific export steps #236
• Platform roadmap: Platform roadmap research: Steam Deck, Android, iOS, Web #100