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.

src/components/rpg-entry/rpgEntryWorldPresentation.ts

@@ -2,6 +2,7 @@ import type { BigFishWorkSummary } from '../../../packages/shared/src/contracts/
 import type { BabyObjectMatchDraft } from '../../../packages/shared/src/contracts/edutainmentBabyObject';
 import { BABY_OBJECT_MATCH_EDUTAINMENT_TAG } from '../../../packages/shared/src/contracts/edutainmentBabyObject';
 import type {
+  Match3DGeneratedBackgroundAsset,
   Match3DGeneratedItemAsset,
   Match3DWorkSummary,
 } from '../../../packages/shared/src/contracts/match3dWorks';
@@ -117,6 +118,7 @@ export type PlatformMatch3DGalleryCard = {
   backgroundPrompt?: string | null;
   backgroundImageSrc?: string | null;
   backgroundImageObjectKey?: string | null;
+  generatedBackgroundAsset?: Match3DGeneratedBackgroundAsset | null;
   generatedItemAssets?: Match3DGeneratedItemAsset[];
 };
 
@@ -298,6 +300,7 @@ export function mapMatch3DWorkToPlatformGalleryCard(
     backgroundPrompt: work.backgroundPrompt ?? null,
     backgroundImageSrc: work.backgroundImageSrc ?? null,
     backgroundImageObjectKey: work.backgroundImageObjectKey ?? null,
+    generatedBackgroundAsset: work.generatedBackgroundAsset ?? null,
     generatedItemAssets: work.generatedItemAssets ?? [],
   };
 }