feat(web,engine): custom style — image upload, AI-extract prompt, painter ref

自定义画风入口里加上传按钮：客户端把图缩到 512px webp(base64)，传到新
路由 /api/parse-style-image，vision LLM 解析成英文 style prompt 回填 textarea；
图本身随 sessionStorage → /api/start → Session.styleReferenceImage 透传，
painter.collectReferenceImages 把它置于 slot 0，整局每一幕都作为 reference
图锚定画风（brush / color / mood），比 priorScene 优先级更高。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/play/page.tsx

@@ -500,7 +500,11 @@ function PlayInner() {
     const presetId = params.get("preset");
     const isCustom = params.get("custom") === "1";
 
-    let livePayload: { worldSetting: string; styleGuide: string } | null = null;
+    let livePayload: {
+      worldSetting: string;
+      styleGuide: string;
+      styleReferenceImage?: string;
+    } | null = null;
     if (!cardName) {
       if (presetId) {
         const p = PRESETS.find((x) => x.id === presetId);
@@ -513,8 +517,13 @@ function PlayInner() {
               worldSetting: string;
               styleGuide: string;
               audioEnabled?: boolean;
+              styleReferenceImage?: string;
+            };
+            livePayload = {
+              worldSetting: parsed.worldSetting,
+              styleGuide: parsed.styleGuide,
+              styleReferenceImage: parsed.styleReferenceImage || undefined,
             };
-            livePayload = { worldSetting: parsed.worldSetting, styleGuide: parsed.styleGuide };
             // audioEnabled 已在 useState 初始化时反向投射到 muted；这里无需再额外存。
           } catch {
             livePayload = null;
@@ -531,6 +540,11 @@ function PlayInner() {
     type PrebakedFirstAct = StartResponse & {
       worldSetting: string;
       styleGuide: string;
+      // Live /api/start path tags this on after the response (prebaked card
+      // JSONs never have one — they were rendered at build time without any
+      // user-uploaded reference). Carried into Session so /api/scene's painter
+      // anchors the same style image on every subsequent scene.
+      styleReferenceImage?: string;
       cardName?: string;
       cardTitle?: string;
       cardGender?: string;
@@ -554,7 +568,14 @@ function PlayInner() {
           }
           const data = (await r.json()) as StartResponse;
           // Live /api/start doesn't echo ws/sg back — splice in what we sent.
-          return { ...data, worldSetting: livePayload!.worldSetting, styleGuide: livePayload!.styleGuide };
+          // styleReferenceImage is similarly not in StartResponse; tag it on so
+          // the session we build below carries it for every /api/scene call.
+          return {
+            ...data,
+            worldSetting: livePayload!.worldSetting,
+            styleGuide: livePayload!.styleGuide,
+            styleReferenceImage: livePayload!.styleReferenceImage,
+          };
         });
 
     fetchStart
@@ -577,6 +598,7 @@ function PlayInner() {
           ],
           characters: data.characters,
           storyState: data.storyState,
+          styleReferenceImage: data.styleReferenceImage,
         };
         visitedBeatsRef.current = [data.scene.entryBeatId];
         setSession(initial);