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