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/ww_core/orchestrator/chain/nodes.py

@@ -20,6 +20,7 @@ from ww_agents import AgentSpec
 
 from .._protocols import GatewayRun
 from ..collect import collect_reviews
+from ..generation_node import build_continuation_context
 from ..review_node import build_review_context, run_review
 from ..state import ChapterState
 from ..write_node import build_write_request
@@ -27,6 +28,9 @@ from .state import ChainState
 
 log = structlog.get_logger(__name__)
 
+#: 续写式链 key：写章以上一章已验收正文末尾作前文引子（F2）。
+CHAIN_CONTINUE_VOLUME = "continue_volume"
+
 # ---- 注入缝（图工厂经默认参绑定，单测直接注 fake）----
 
 #: `session -> Repo`：节点自建短事务里从 session 造 repo（仿端点依赖工厂）。
@@ -62,7 +66,10 @@ class DraftView(Protocol):
 
 
 class ChapterDraftRepo(Protocol):
-    """章草稿 repo 最小依赖：write 落稿 `save_draft` + review 重读 `get_draft`（只 flush）。"""
+    """章草稿 repo 最小依赖：write 落稿 `save_draft` + review 重读 `get_draft`（只 flush）。
+
+    续写式链（`continue_volume`）写章另需 `latest_accepted`——读上一章已验收终稿正文作前文引子。
+    """
 
     async def save_draft(
         self, project_id: uuid.UUID, chapter_no: int, *, text: str, volume: int = 1
@@ -70,6 +77,8 @@ class ChapterDraftRepo(Protocol):
 
     async def get_draft(self, project_id: uuid.UUID, chapter_no: int) -> DraftView | None: ...
 
+    async def latest_accepted(self, project_id: uuid.UUID, chapter_no: int) -> DraftView | None: ...
+
 
 class ReviewRecordRepo(Protocol):
     """review/accept 节点对审稿 repo 的最小依赖：collect 留痕 + accept 重读最近审稿。"""
@@ -126,20 +135,30 @@ async def write_chapter(
     project_id = state["project_id"]
     chapter_no = state["current_chapter_no"]
     user_id = state["user_id"]
+    is_continuation = state.get("chain_key") == CHAIN_CONTINUE_VOLUME
     async with session_factory() as session:
         gateway = await gateway_builder(session)
         repos = memory_repos_factory(session)
         context = await assemble(repos, project_id, chapter_no)
+        chapter_repo = chapter_repo_factory(session)
+        # 续写模式：volatile 改写为「设定 + 上一章已验收正文末尾」（不变量 #9：仅改断点后块，
+        # stable_core 仍进缓存前缀）。前文经注入的 chapter_repo 重读（core 不 import apps/api）。
+        if is_continuation:
+            prior_text = await _read_prior_accepted(chapter_repo, project_id, chapter_no)
+            volatile = build_continuation_context(
+                project_context=context.volatile, prior_text=prior_text
+            )
+        else:
+            volatile = context.volatile
         req = build_write_request(
             stable_core=context.stable_core,
-            volatile=context.volatile,
+            volatile=volatile,
             user_id=user_id,
             project_id=project_id,
         )
         # 收集版：非流式，一次 run 拿全文（链批量量产不走 SSE）。
         collect_req = req.model_copy(update={"stream": False})
         resp = await gateway.run(collect_req)
-        chapter_repo = chapter_repo_factory(session)
         await chapter_repo.save_draft(project_id, chapter_no, text=resp.text)
         await _commit(session)
     log.info(
@@ -295,6 +314,19 @@ async def _read_draft(
     return view.content if view is not None else ""
 
 
+async def _read_prior_accepted(
+    chapter_repo: ChapterDraftRepo, project_id: uuid.UUID, chapter_no: int
+) -> str:
+    """续写式链：读上一章（chapter_no-1）已验收终稿正文作前文引子（不变量 #1/#4）。
+
+    第一章（前一章号 < 1）或前一章未验收 → 空串，由 `build_continuation_context` 降级占位。
+    """
+    if chapter_no <= 1:
+        return ""
+    view = await chapter_repo.latest_accepted(project_id, chapter_no - 1)
+    return view.content if view is not None else ""
+
+
 async def _commit(session: CommitSession) -> None:
     """提交短事务（session 满足 `CommitSession`：只需 `commit()`）。"""
     await session.commit()
@@ -307,6 +339,7 @@ def has_conflicts(state: ChainState) -> bool:
 
 
 __all__ = [
+    "CHAIN_CONTINUE_VOLUME",
     "AcceptChapterOp",
     "AssembleContext",
     "AssembleFn",