feat(writer): add WriterGenerationError

src/genesis/writer/exceptions.py

@@ -0,0 +1,6 @@
+"""Writer 子系统异常。"""
+from __future__ import annotations
+
+
+class WriterGenerationError(Exception):
+    """LLM 章节生成失败（引擎 status 非 ok/fallback、或解析耗尽）。"""

tests/test_phase5_exceptions.py

@@ -0,0 +1,9 @@
+import pytest
+from genesis.writer.exceptions import WriterGenerationError
+
+
+def test_writer_generation_error_is_exception():
+    with pytest.raises(WriterGenerationError):
+        raise WriterGenerationError("生成失败")
+    with pytest.raises(Exception):
+        raise WriterGenerationError("x")