feat: M1 — 立项→写章草稿(SSE)→自动保存；连一家 provider

- 薄自建 LLM 网关：OpenAI 兼容适配器(DeepSeek) + instructor 结构化输出 + usage_ledger 记账 + 档位路由
- 记忆服务 assemble：确定性选择(显式+主角+近况) + 渲染卡 + 缓存断点(中性文本)
- LangGraph 写章节点 + Postgres checkpointer + SSE 归一(token/done/error)
- API：立项 + 写章 draft(SSE) + PUT 自动保存 + 提供商凭据(Fernet 加密/测试连接)
- 前端：AppShell + 作品库 + 5 步立项向导 + 写作工作台(流式打字机+自动保存) + 设置页
- M1 E2E：真实 DB + mock 网关零 token 走通闭环

apps/web/lib/stream/sse.ts

@@ -0,0 +1,106 @@
+// SSE 帧解析 + 草稿流归一（对齐 C3 / ARCH §7.3）。
+// 帧格式：`event:<token|done|error>\ndata:<json>\n\n`。
+// 纯逻辑，便于单测（不依赖浏览器/DOM）。
+
+export interface TokenEvent {
+  event: "token";
+  data: { text: string };
+}
+export interface DoneEvent {
+  event: "done";
+  data: { length: number };
+}
+export interface ErrorEvent {
+  event: "error";
+  data: { code: string; message: string; request_id?: string | null };
+}
+export type SseEvent = TokenEvent | DoneEvent | ErrorEvent;
+
+const KNOWN_EVENTS = new Set(["token", "done", "error"]);
+
+// 把一个完整 SSE 块（多行）解析成事件；无法解析则返回 null（跳过）。
+export function parseSseBlock(block: string): SseEvent | null {
+  let event = "";
+  const dataLines: string[] = [];
+  for (const rawLine of block.split("\n")) {
+    const line = rawLine.replace(/\r$/, "");
+    if (line.startsWith(":")) continue; // 注释/心跳
+    const sep = line.indexOf(":");
+    if (sep === -1) continue;
+    const field = line.slice(0, sep);
+    const value = line.slice(sep + 1).replace(/^ /, "");
+    if (field === "event") event = value;
+    else if (field === "data") dataLines.push(value);
+  }
+  if (!KNOWN_EVENTS.has(event) || dataLines.length === 0) return null;
+  let data: unknown;
+  try {
+    data = JSON.parse(dataLines.join("\n"));
+  } catch {
+    return null;
+  }
+  return { event, data } as SseEvent;
+}
+
+// 增量缓冲：吃进一段文本，吐出已完成的事件块（以空行分隔），保留未完成尾部。
+export class SseFrameBuffer {
+  private buf = "";
+
+  push(chunk: string): SseEvent[] {
+    this.buf += chunk;
+    const events: SseEvent[] = [];
+    let idx: number;
+    // 块之间以空行（\n\n，兼容 \r\n\r\n）分隔。
+    while ((idx = this.findBoundary(this.buf)) !== -1) {
+      const block = this.buf.slice(0, idx.valueOf());
+      this.buf = this.buf.slice(this.boundaryEnd(this.buf, idx));
+      const evt = parseSseBlock(block);
+      if (evt) events.push(evt);
+    }
+    return events;
+  }
+
+  private findBoundary(s: string): number {
+    const a = s.indexOf("\n\n");
+    const b = s.indexOf("\r\n\r\n");
+    if (a === -1) return b;
+    if (b === -1) return a;
+    return Math.min(a, b);
+  }
+
+  private boundaryEnd(s: string, idx: number): number {
+    return s.startsWith("\r\n\r\n", idx) ? idx + 4 : idx + 2;
+  }
+}
+
+export type StreamPhase = "idle" | "streaming" | "done" | "error" | "aborted";
+
+export interface StreamState {
+  phase: StreamPhase;
+  text: string;
+  error: { code: string; message: string; request_id?: string | null } | null;
+}
+
+export const initialStreamState: StreamState = {
+  phase: "idle",
+  text: "",
+  error: null,
+};
+
+// 纯 reducer：把单个事件折叠进状态（打字机文本累积）。
+export function reduceStream(state: StreamState, event: SseEvent): StreamState {
+  switch (event.event) {
+    case "token":
+      return {
+        ...state,
+        phase: "streaming",
+        text: state.text + event.data.text,
+      };
+    case "done":
+      return { ...state, phase: "done" };
+    case "error":
+      return { ...state, phase: "error", error: event.data };
+    default:
+      return state;
+  }
+}