Update Match3D/image-generation docs & code

Adds/updates documentation, assets and implementation for Match3D and puzzle image generation workflows. Key changes: decision logs and pitfalls updated to prefer VectorEngine Gemini for Match3D material sheets and to require edits (multipart) for 1:1 container reference images; guidance added for when to use APIMart vs VectorEngine. .env.example clarified APIMart/Responses config. Many new public assets and PPT visuals added. Code changes across frontend and backend: updated shared contracts, server-rs match3d/puzzle/image-generation handlers, VectorEngine/OpenAI image generation clients, and multiple React components/tests to handle UI/background/container image signing, edits workflow, and puzzle UI background resolution. Added src/services/puzzle-runtime/puzzleUiBackgroundSource.ts and related test updates. Includes notes about multipart HTTP/1.1 requirement and test/verification commands in docs.

packages/shared/src/contracts/puzzleRuntimeSession.ts

@@ -58,6 +58,7 @@ export interface PuzzleRuntimeLevelSnapshot {
   themeTags: string[];
   coverImageSrc: string | null;
   uiBackgroundImageSrc?: string | null;
+  uiBackgroundImageObjectKey?: string | null;
   backgroundMusic?: CreationAudioAsset | null;
   board: PuzzleBoardSnapshot;
   status: PuzzleRuntimeLevelStatus;