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.

server-rs/crates/shared-contracts/src/match3d_works.rs

@@ -64,7 +64,11 @@ pub struct PersistMatch3DGeneratedModelResponse {
 pub struct GenerateMatch3DCoverImageRequest {
     pub prompt: String,
     #[serde(default)]
+    pub uploaded_image_src: Option<String>,
+    #[serde(default)]
     pub reference_image_src: Option<String>,
+    #[serde(default)]
+    pub reference_image_srcs: Vec<String>,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
@@ -92,10 +96,28 @@ pub struct GenerateMatch3DBackgroundImageResponse {
     pub prompt: String,
 }
 
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+#[serde(rename_all = "camelCase")]
+pub struct GenerateMatch3DContainerImageRequest {
+    pub prompt: String,
+}
+
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+#[serde(rename_all = "camelCase")]
+pub struct GenerateMatch3DContainerImageResponse {
+    pub item: Match3DWorkProfileResponse,
+    pub container_image_src: String,
+    pub container_image_object_key: String,
+    pub generated_background_asset: Match3DGeneratedBackgroundAssetResponse,
+    pub prompt: String,
+}
+
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
 #[serde(rename_all = "camelCase")]
 pub struct GenerateMatch3DItemAssetsRequest {
     pub item_names: Vec<String>,
+    #[serde(default)]
+    pub mode: Option<String>,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]

server-rs/crates/shared-contracts/src/puzzle_runtime.rs

@@ -120,6 +120,8 @@ pub struct PuzzleRuntimeLevelSnapshotResponse {
     #[serde(default)]
     pub ui_background_image_src: Option<String>,
     #[serde(default)]
+    pub ui_background_image_object_key: Option<String>,
+    #[serde(default)]
     pub background_music: Option<CreationAudioAsset>,
     pub board: PuzzleBoardSnapshotResponse,
     pub status: String,