feat(ux): T4-b 后端 — draft 端点接收本章指令 directive 直通 assemble(volatile)

assemble/_build_volatile 加 directive 参数（仅入 volatile，守缓存前缀不变量 #9）；stream_draft 加可选 body {directive} 透传；不持久化、无迁移；directive 长度入 log 不记原文。新 DraftStreamRequest schema + 重生成 TS 客户端。后端门禁绿: ruff/mypy/pytest。契约记 memory/contracts.md。

apps/api/tests/test_projects.py

@@ -196,6 +196,33 @@ async def test_draft_stream_yields_sse_tokens_and_done() -> None:
     assert gateway.requests[0].tier == "writer"
 
 
+@pytest.mark.asyncio
+async def test_draft_stream_threads_directive_into_volatile() -> None:
+    gateway = FakeWriterGateway(chunks=["正文"])
+    client, _, _, _ = _make_client(gateway=gateway)
+    pid = uuid.uuid4()
+    async with client:
+        resp = await client.post(
+            f"/projects/{pid}/chapters/1/draft", json={"directive": "多写战斗"}
+        )
+    assert resp.status_code == 200
+    assert len(gateway.requests) == 1
+    # 指令直通 assemble→volatile（write 节点把 volatile 放进 LlmRequest.input）。
+    assert "多写战斗" in str(gateway.requests[0].input)
+
+
+@pytest.mark.asyncio
+async def test_draft_stream_backward_compatible_without_body() -> None:
+    gateway = FakeWriterGateway(chunks=["正文"])
+    client, _, _, _ = _make_client(gateway=gateway)
+    pid = uuid.uuid4()
+    async with client:
+        resp = await client.post(f"/projects/{pid}/chapters/1/draft")
+    assert resp.status_code == 200
+    assert len(gateway.requests) == 1
+    assert "本章指令" not in str(gateway.requests[0].input)
+
+
 @pytest.mark.asyncio
 async def test_draft_stream_maps_error_to_sse_error_event() -> None:
     from ww_shared import AppError

apps/api/ww_api/routers/projects.py

@@ -53,6 +53,7 @@ from ww_api.schemas.projects import (
     AcceptResponse,
     DraftResponse,
     DraftSaveRequest,
+    DraftStreamRequest,
     DraftView,
     ProjectCreateRequest,
     ProjectListResponse,
@@ -240,11 +241,17 @@ async def stream_draft(
     gateway: GatewayDep,
     injection_repo: InjectionRepoDep,
     session: Annotated[AsyncSession, Depends(get_session)],
+    body: DraftStreamRequest | None = None,
 ) -> StreamingResponse:
-    """流式写章草稿：组装记忆（含作者注入覆盖）→ 网关流 → 归一为 SSE 事件 → text/event-stream。"""
+    """流式写章草稿：组装记忆（含作者注入覆盖 + 本章指令）→ 网关流 → 归一 SSE → event-stream。
+
+    `body.directive`（可选，T4-b）是临时本章指令，直通 assemble→volatile（不持久化）；
+    无 body 的旧调用方仍可用（向后兼容）。
+    """
     request_id = getattr(request.state, "request_id", None)
+    directive = body.directive if body else None
     override = await injection_repo.get(project_id, chapter_no)
-    context = await assemble(repos, project_id, chapter_no, override=override)
+    context = await assemble(repos, project_id, chapter_no, override=override, directive=directive)
     log.info(
         "draft_stream_start",
         project_id=str(project_id),
@@ -252,6 +259,7 @@ async def stream_draft(
         request_id=request_id,
         stable_core_len=len(context.stable_core),
         volatile_len=len(context.volatile),
+        directive_len=len(body.directive) if body and body.directive else 0,
     )
 
     deltas = stream_chapter_draft(

apps/api/ww_api/schemas/projects.py

@@ -42,6 +42,16 @@ class ProjectListResponse(BaseModel):
     projects: list[ProjectResponse] = Field(default_factory=list)
 
 
+class DraftStreamRequest(BaseModel):
+    """POST /projects/:id/chapters/:no/draft：本章生成的可选输入（T4-b）。
+
+    `directive` = 作者本章指令（覆盖/补充大纲节拍）。临时输入，不持久化、无迁移；
+    后端只入 volatile（守缓存前缀不变量 #9）。无 body 时退化为纯按大纲生成（向后兼容）。
+    """
+
+    directive: str | None = None
+
+
 class DraftSaveRequest(BaseModel):
     """PUT /projects/:id/chapters/:no/draft：自动保存草稿正文。"""