Implement comprehensive image stack functionality for the room system, allowing AI agents to add, organize, and persist images in the workspace. New features: - Image stacking on surfaces with LRU texture caching (max 50 textures/50MB) - Persistent image storage with UUID-based filenames - Type-safe ImagePath validation (absolute paths, format checking) - Backend file validation before IPC (size, format, existence checks) - Room schema v1.1 upgrade from v1.0 with migration support - MCP tool (add_image) for agent interaction Infrastructure: - ImageCache with Drop trait for proper GPU resource cleanup - ImageStorage module for atomic file operations - ImageStack and ImageRef interactable types - Enhanced error types with detailed messages - Integration test for end-to-end workflow Backend changes: - validateImagePathAndFile() with comprehensive checks - RoomAction.AddImage with type-safe ImagePath - ImagePath branded type for compile-time safety Files added: - game/src/room/image_cache.rs (LRU cache with Drop) - game/src/room/image_storage.rs (persistence layer) - game/src/persistence/fixtures/room_v1_1.json (migration test) All 204 tests passing with zero clippy warnings.