chore: Phase3 Word 解析收尾（尾随换行/import 排序/契约注释）

src/genesis/parsers/source_aggregator.py

@@ -28,6 +28,8 @@ class SourceParser:
         write_instruction_paths: list[str | Path] | None = None,
         rule_paths: list[str | Path] | None = None,
     ) -> StructuredSource:
+        """扩展名校验先于存在性校验（不存在的文件若扩展名未知将抛出 ValueError 而非 FileNotFoundError）。"""
+
         requirement_paths = requirement_paths or []
         write_instruction_paths = write_instruction_paths or []
         rule_paths = rule_paths or []
@@ -70,4 +72,4 @@ class SourceParser:
             image_analyses=[],
             existing_system=None,
             comments=comments,
-        )
+        )