feat(orchestrator): 续写链前一章无已验收终稿发 warning——静默丢前文引子可观测（CR-L1）

packages/core/ww_core/orchestrator/chain/nodes.py

@@ -325,7 +325,17 @@ async def _read_prior_accepted(
     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 ""
+    if view is None:
+        # 静默降级须可观测（CR-L1）：续写式链本章续接的前一章尚无已验收终稿——续写会丢失
+        # 前文引子（降级为 build_continuation_context 的占位）。发 warning 便于查错，勿静默吞。
+        log.warning(
+            "chain_prior_accepted_missing",
+            project_id=str(project_id),
+            chapter_no=chapter_no,
+            prior_chapter_no=chapter_no - 1,
+        )
+        return ""
+    return view.content
 
 
 async def _commit(session: CommitSession) -> None: