feat(chain): F2 续写式链 continue_volume——上一章 accepted 正文作前文引子

- core write_chapter 支持续写模式：chain_key==continue_volume 时经注入的
  chapter_repo.latest_accepted 读上一章已验收终稿，build_continuation_context
  改写 volatile（仅断点后块，守不变量 #9；core 不 import apps/api）。
- ChapterDraftRepo 协议加 latest_accepted；draft_volume 行为不变（无回归）。
- apps/api SUPPORTED_CHAINS 加 continue_volume；run 端点透传 chain_key 选模式
  （chain_runner 经 initial_chain_state 把 chain_key 落入 ChainState）。
- 单测（mock 网关+MemorySaver+fake session）：续写第二章请求含第一章正文；
  draft_volume 不注入前文；端点 202 透传 chain_key。守不变量 #1/#5。

packages/core/tests/test_chain_graph.py

@@ -64,13 +64,17 @@ class FakeChainGateway:
         self._by_schema = by_schema
         self.write_calls = 0
         self.review_calls = 0
+        self.write_inputs: list[str] = []
 
     async def run(self, req: LlmRequest) -> LlmResponse:
         schema: type[BaseModel] | None = req.output_schema
         if schema is None:
             self.write_calls += 1
+            self.write_inputs.append(req.input if isinstance(req.input, str) else "")
+            # 每次写章产不同正文（便于断言下一章续写注入了上一章正文）。
+            draft = f"{self._draft}#{self.write_calls}"
             return LlmResponse(
-                text=self._draft,
+                text=draft,
                 parsed=None,
                 usage=_usage(),
                 served_by=ServedBy(provider="fake", model="fake"),
@@ -93,10 +97,11 @@ class FakeDraftView:
 
 
 class FakeChapterRepo:
-    """内存章草稿 repo：save_draft 存正文，get_draft 回读。"""
+    """内存章草稿 repo：save_draft 存正文，get_draft 回读，latest_accepted 读已验收正文。"""
 
-    def __init__(self, store: dict[int, str]) -> None:
+    def __init__(self, store: dict[int, str], accepted: dict[int, str] | None = None) -> None:
         self._store = store
+        self._accepted = accepted if accepted is not None else {}
 
     async def save_draft(
         self, project_id: uuid.UUID, chapter_no: int, *, text: str, volume: int = 1
@@ -108,6 +113,10 @@ class FakeChapterRepo:
         content = self._store.get(chapter_no)
         return FakeDraftView(content) if content is not None else None
 
+    async def latest_accepted(self, project_id: uuid.UUID, chapter_no: int) -> Any:
+        content = self._accepted.get(chapter_no)
+        return FakeDraftView(content) if content is not None else None
+
 
 class FakeReviewRecord:
     def __init__(self, chapter_no: int, conflicts: list[dict[str, Any]]) -> None:
@@ -177,6 +186,7 @@ def _empty_by_schema() -> dict[type, BaseModel]:
 def _make_harness(*, conflicts: list[Conflict]) -> dict[str, Any]:
     """造一套共享内存态 + 工厂闭包 + accept 间谍，供图/节点测试复用。"""
     draft_store: dict[int, str] = {}
+    accepted_store: dict[int, str] = {}
     review_store: dict[int, list[FakeReviewRecord]] = {}
     accepted: list[dict[str, Any]] = []
 
@@ -192,7 +202,7 @@ def _make_harness(*, conflicts: list[Conflict]) -> dict[str, Any]:
         return object()  # assemble 被 fake 替换，不实际用 repos
 
     def chapter_repo_factory(session: Any) -> Any:
-        return FakeChapterRepo(draft_store)
+        return FakeChapterRepo(draft_store, accepted_store)
 
     def review_repo_factory(session: Any) -> Any:
         return FakeReviewRepo(review_store)
@@ -208,6 +218,9 @@ def _make_harness(*, conflicts: list[Conflict]) -> dict[str, Any]:
         decisions: list[Any],
     ) -> None:
         accepted.append({"chapter_no": chapter_no, "decisions": list(decisions)})
+        # 仿真验收落库：把本章草稿晋升为 accepted（供下一章续写读前文）。
+        if chapter_no in draft_store:
+            accepted_store[chapter_no] = draft_store[chapter_no]
 
     return {
         "gateway": gateway,
@@ -220,6 +233,7 @@ def _make_harness(*, conflicts: list[Conflict]) -> dict[str, Any]:
         "accept_op": accept_op,
         "accepted": accepted,
         "draft_store": draft_store,
+        "accepted_store": accepted_store,
         "review_store": review_store,
     }
 
@@ -330,7 +344,7 @@ async def test_write_chapter_saves_collected_draft() -> None:
     )
 
     assert out == {}  # state 不变（正文在 DB）
-    assert h["draft_store"][1] == "第 N 章正文。"
+    assert h["draft_store"][1] == "第 N 章正文。#1"
     # 收集版：write 走非流式 run
     assert h["gateway"].write_calls == 1
 
@@ -378,3 +392,80 @@ async def test_chain_interrupts_on_conflict_then_resumes() -> None:
     assert final["written"] == [1]
     assert [a["chapter_no"] for a in h["accepted"]] == [1]
     assert h["accepted"][0]["decisions"] == decisions  # 裁决透传给 accept_op
+
+
+# ---- F2 续写式链 continue_volume ----
+
+
+async def test_write_chapter_draft_volume_omits_prior_text() -> None:
+    """draft_volume（默认）写章不注入前文——仍走 assemble 的 volatile（无回归）。"""
+    h = _make_harness(conflicts=[])
+    h["accepted_store"][0] = "上一章正文末尾片段。"  # 即使有前文也不应注入
+    state = initial_chain_state(
+        project_id=PROJECT, user_id=USER, start_chapter_no=1, count=1
+    )  # 默认 chain_key=draft_volume
+
+    await write_chapter(
+        state,
+        gateway_builder=h["gateway_builder"],
+        session_factory=h["session_factory"],
+        memory_repos_factory=h["memory_repos_factory"],
+        chapter_repo_factory=h["chapter_repo_factory"],
+        assemble=h["assemble"],
+    )
+
+    write_input = h["gateway"].write_inputs[0]
+    assert write_input == "## 写作指令\n写第 N 章"  # 原 assemble volatile，未改写
+    assert "上一章正文末尾片段" not in write_input
+
+
+async def test_write_chapter_continue_volume_injects_prior_accepted_text() -> None:
+    """continue_volume 写第 2 章时，注入第 1 章 accepted 正文（经 build_continuation_context）。"""
+    h = _make_harness(conflicts=[])
+    h["accepted_store"][1] = "第一章已验收的正文末尾。"
+    state = initial_chain_state(
+        project_id=PROJECT,
+        user_id=USER,
+        start_chapter_no=2,
+        count=1,
+        chain_key="continue_volume",
+    )
+
+    await write_chapter(
+        state,
+        gateway_builder=h["gateway_builder"],
+        session_factory=h["session_factory"],
+        memory_repos_factory=h["memory_repos_factory"],
+        chapter_repo_factory=h["chapter_repo_factory"],
+        assemble=h["assemble"],
+    )
+
+    write_input = h["gateway"].write_inputs[0]
+    assert "第一章已验收的正文末尾。" in write_input
+    assert "前文正文" in write_input  # build_continuation_context 的小节标题
+
+
+async def test_continue_volume_chain_second_chapter_sees_first_chapter_text() -> None:
+    """全图 E2E：continue_volume 两章——第 2 章请求上下文含第 1 章正文（不变量 #1/#5）。"""
+    h = _make_harness(conflicts=[])
+    graph = _build(h, MemorySaver())
+    config: RunnableConfig = {"configurable": {"thread_id": "chain-continue"}}
+    initial = initial_chain_state(
+        project_id=PROJECT,
+        user_id=USER,
+        start_chapter_no=1,
+        count=2,
+        chain_key="continue_volume",
+    )
+
+    final = await graph.ainvoke(initial, config=config)
+
+    assert "__interrupt__" not in final
+    assert final["written"] == [1, 2]
+    # 第 1 章写出的正文（accept_op 晋升为 accepted）应出现在第 2 章的写章请求里。
+    first_chapter_text = h["accepted_store"][1]
+    second_write_input = h["gateway"].write_inputs[1]
+    assert first_chapter_text in second_write_input
+    # 第 1 章请求无前文（前一章 0 不存在）→ 降级占位，不报错。
+    first_write_input = h["gateway"].write_inputs[0]
+    assert "前文正文" in first_write_input