feat(engine): auto-select art style via parallel LLM call

When user picks "自动", the client sends styleGuide="auto" to the
server. The orchestrator then runs a lightweight style-selector LLM
call in parallel with the Architect — both only depend on worldSetting,
so there is zero added latency. The selector picks the best-matching
preset from STYLE_MAP based on genre, mood, and setting.

Also moves STYLE_MAP from page.tsx to lib/options.ts so it can be
shared between client and server.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/engine/agents/styleSelector.ts

@@ -0,0 +1,37 @@
+import { chat } from "@infiplot/ai-client";
+import type { ProviderConfig } from "@infiplot/types";
+import { STYLE_MAP } from "@/lib/options";
+
+const STYLE_NAMES = Object.keys(STYLE_MAP);
+
+const SYSTEM = `You are an art director for a visual novel. Given the story premise, pick the single best-matching art style from the list below. Consider the genre, mood, setting, and target audience.
+
+Available styles:
+${STYLE_NAMES.map((s) => `- ${s}`).join("\n")}
+
+Reply with ONLY the style name, nothing else. If uncertain, default to 吉卜力.`;
+
+export async function selectStyle(
+  textConfig: ProviderConfig,
+  worldSetting: string,
+): Promise<string> {
+  const result = await chat(
+    textConfig,
+    [
+      { role: "system", content: SYSTEM },
+      { role: "user", content: worldSetting },
+    ],
+    { temperature: 0, tag: "styleSelector" },
+  );
+
+  const picked = result.trim();
+  if (STYLE_MAP[picked]) {
+    return STYLE_MAP[picked];
+  }
+  const fuzzy = STYLE_NAMES.find((s) => picked.includes(s) || s.includes(picked));
+  if (fuzzy) {
+    return STYLE_MAP[fuzzy]!;
+  }
+  console.warn(`[styleSelector] unrecognized style "${picked}", falling back to 吉卜力`);
+  return STYLE_MAP["吉卜力"]!;
+}

lib/engine/orchestrator.ts

@@ -14,6 +14,7 @@ import type {
 } from "@infiplot/types";
 import { coerceOrientation } from "@infiplot/types";
 import { runArchitect } from "./agents/architect";
+import { selectStyle } from "./agents/styleSelector";
 import { directInsertBeat, directScene } from "./director";
 import { synthesizeBeat } from "./voice";
 import { interpret } from "./vision";
@@ -52,16 +53,25 @@ export async function startSession(
     orientation: coerceOrientation(req.orientation),
   };
 
-  // Stage 0 — Architect: expand the terse world/style prompt into a story
-  // bible BEFORE the first scene. Serial by necessity (the opening Writer
-  // reads session.storyState), but it gives the whole story a spine from beat
-  // one — the latency is offset by the director's portrait/voice overlap win.
+  // Stage 0 — Architect (+ optional auto style selection, in parallel).
+  // Both only depend on worldSetting, so they run concurrently.
   console.log(
     `[start] worldSetting (${session.worldSetting.length} chars):\n${session.worldSetting}`,
   );
+  const isAutoStyle = session.styleGuide === "auto";
   const tArchitect = Date.now();
-  session.storyState = await runArchitect(config.text, session);
-  tlog("[start] Architect", tArchitect);
+  const [architectResult, autoStyleGuide] = await Promise.all([
+    runArchitect(config.text, session),
+    isAutoStyle
+      ? selectStyle(config.text, session.worldSetting)
+      : Promise.resolve(null),
+  ]);
+  session.storyState = architectResult;
+  if (isAutoStyle && autoStyleGuide) {
+    session.styleGuide = autoStyleGuide;
+    console.log(`[start] auto-selected style: ${autoStyleGuide.slice(0, 60)}…`);
+  }
+  tlog("[start] Architect" + (isAutoStyle ? " + StyleSelector" : ""), tArchitect);
   console.log(
     `[start] storyBible: logline="${session.storyState.logline}" | genreTags="${session.storyState.genreTags}" | synopsis="${session.storyState.synopsis}"`,
   );