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>

lib/engine/agents/painter.ts

@@ -47,6 +47,13 @@ export type PainterInput = {
    * with character refs, capped at 4 total per Runware spec.
    */
   priorSceneImage?: string;
+  /**
+   * User-uploaded style reference (data URL base64). When set, it takes the
+   * highest-priority slot in referenceImages so the painting STYLE (brush /
+   * color / mood) of the user's image is anchored across every scene this
+   * session paints — even before any priorScene exists.
+   */
+  styleReferenceImage?: string;
 };
 
 // Pick the references we send to Runware as `referenceImages`. Priority:
@@ -59,14 +66,22 @@ export function collectReferenceImages(
   characters: Character[],
   entryBeat: Beat | undefined,
   priorSceneImage: string | undefined,
+  styleReferenceImage?: string,
 ): string[] {
   const refs: string[] = [];
   const seen = new Set<string>();
 
-  // Slot 0 — prior scene image for spatial continuity. Goes first because
-  // backdrop drift is the most jarring discontinuity across same-sceneKey
-  // scenes; character drift is partially masked by character archetype text
-  // in the prompt anyway.
+  // Slot 0 — user-uploaded style reference image, if any. Goes first because
+  // it anchors the whole-session painting STYLE (brush / color / mood) that
+  // the user explicitly chose. priorScene continuity comes second; character
+  // archetypes are partially covered by the prompt text anyway.
+  if (styleReferenceImage) {
+    refs.push(styleReferenceImage);
+  }
+
+  // Slot N — prior scene image for spatial continuity. Backdrop drift is the
+  // next-most jarring discontinuity across same-sceneKey scenes; character
+  // drift is partially masked by character archetype text in the prompt.
   if (priorSceneImage) {
     refs.push(priorSceneImage);
   }
@@ -140,6 +155,7 @@ export async function runPainter(
     input.onStageCharacters,
     entryBeat,
     input.priorSceneImage,
+    input.styleReferenceImage,
   );
 
   // Tier A — with referenceImages (priorSceneImage + character portraits).

lib/engine/director.ts

@@ -327,6 +327,7 @@ export async function directScene(
       styleGuide: session.styleGuide,
       onStageCharacters,
       priorSceneImage: priorSceneReference,
+      styleReferenceImage: session.styleReferenceImage,
     },
     entryBeat,
   );

lib/engine/orchestrator.ts

@@ -47,6 +47,7 @@ export async function startSession(
     styleGuide: req.styleGuide.trim(),
     history: [],
     characters: [],
+    styleReferenceImage: req.styleReferenceImage?.trim() || undefined,
   };
 
   // Stage 0 — Architect: expand the terse world/style prompt into a story