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/spacetime-client/src/mapper.rs

@@ -3817,6 +3817,7 @@ pub(crate) fn map_puzzle_runtime_level_snapshot(
         theme_tags: snapshot.theme_tags,
         cover_image_src: snapshot.cover_image_src,
         ui_background_image_src: snapshot.ui_background_image_src,
+        ui_background_image_object_key: snapshot.ui_background_image_object_key,
         background_music: snapshot.background_music.map(map_puzzle_audio_asset),
         board: map_puzzle_board_snapshot(snapshot.board),
         status: snapshot.status.as_str().to_string(),
@@ -7510,6 +7511,7 @@ pub struct PuzzleRuntimeLevelRecord {
     pub theme_tags: Vec<String>,
     pub cover_image_src: Option<String>,
     pub ui_background_image_src: Option<String>,
+    pub ui_background_image_object_key: Option<String>,
     pub background_music: Option<PuzzleAudioAssetRecord>,
     pub board: PuzzleBoardRecord,
     pub status: String,