fix(writer): 注入表格时渲染 caption 段落

表格 Block 的 caption 此前被 DocxInjector 丢弃，导致生成文档缺失表题。
_block_element 现按 [caption, table] 顺序返回元素，并在模板缺少 Caption
样式时回退为普通段落。该修复使 Phase5 headless e2e 的全管线验证得以成立。

src/genesis/writer/docx_injector.py

@@ -89,23 +89,29 @@ class DocxInjector:
 
         # 逆序插入，使最终顺序正确
         for block in reversed(blocks):
-            el = self._block_element(doc, block)
-            parent.insert(para_idx, el)
+            for el in reversed(self._block_element(doc, block)):
+                parent.insert(para_idx, el)
 
-    def _block_element(self, doc: DocxDocument, block: Block):
+    def _block_element(self, doc: DocxDocument, block: Block) -> list:
         if block.kind == "heading":
             p = doc.add_paragraph(block.text, style=f"Heading {block.level}")
-            return p._p
+            return [p._p]
         if block.kind == "table":
+            elements: list = []
+            if block.text:
+                style = "Caption" if "Caption" in doc.styles else None
+                cap = doc.add_paragraph(block.text, style=style)
+                elements.append(cap._p)
             tbl = doc.add_table(rows=0, cols=len(block.rows[0]) if block.rows else 1)
             for r in block.rows:
                 cells = tbl.add_row().cells
                 for i, val in enumerate(r):
                     cells[i].text = str(val)
-            return tbl._tbl  # type: ignore[attr-defined]
+            elements.append(tbl._tbl)  # type: ignore[attr-defined]
+            return elements
         # 默认 paragraph
         p = doc.add_paragraph(block.text)
-        return p._p
+        return [p._p]
 
     def _set_paragraph_text(self, para: Paragraph, text: str) -> None:
         # 清空 run，写入单 run（原型简化；保留段落样式）