feat: 可配置方案引擎通用原型（Mock 检索）

需求未定稿时可先跑通追问/判定/出方案；槽位与判定树外置 YAML，后续可接 Ragflow。

Co-authored-by: haijd <haijd@163.com>

engine/app/static/index.html

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+<!DOCTYPE html>
+<html lang="zh-CN">
+<head>
+  <meta charset="UTF-8" />
+  <meta name="viewport" content="width=device-width, initial-scale=1" />
+  <title>鲜境方案引擎原型</title>
+  <link rel="preconnect" href="https://fonts.googleapis.com" />
+  <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
+  <link href="https://fonts.googleapis.com/css2?family=DM+Sans:wght@400;600;700&family=Noto+Sans+SC:wght@400;500;700&display=swap" rel="stylesheet" />
+  <link rel="stylesheet" href="/static/styles.css" />
+</head>
+<body>
+  <div class="page">
+    <header class="hero">
+      <p class="brand">鲜境物联 · 方案引擎原型</p>
+      <h1>先跑通「追问 → 判定 → 出方案」</h1>
+      <p class="sub">槽位与判定树在 <code>knowledge/*.yaml</code>，需求变了改配置即可。当前检索为 Mock，可后再接 Ragflow。</p>
+    </header>
+
+    <main class="layout">
+      <section class="chat panel">
+        <div id="log" class="log" aria-live="polite"></div>
+        <form id="form" class="composer">
+          <input id="input" type="text" placeholder="描述需求，或直接回答追问…" autocomplete="off" required />
+          <button type="submit">发送</button>
+        </form>
+        <div class="toolbar">
+          <button type="button" id="reset" class="ghost">新会话</button>
+          <span id="meta" class="meta"></span>
+        </div>
+      </section>
+
+      <aside class="side panel">
+        <h2>当前槽位</h2>
+        <pre id="slots">{}</pre>
+        <h2>最近方案</h2>
+        <pre id="scheme">尚未生成</pre>
+      </aside>
+    </main>
+  </div>
+  <script src="/static/app.js"></script>
+</body>
+</html>