feat: layered precise generation for A4 report images

3-phase pipeline to solve LLM prompt overflow from too many OCR elements:
Phase 1 (generate_skeleton): compressed layout schema → skeleton JRXML
Phase 2 (refine_layout): sampled coordinates → pixel-level position tuning
Phase 3 (map_fields): OCR field names → replace $F{field_N} placeholders

Only triggered when layout_schema.total_rows > 0 on initial_generation intent.
Text requests and all other intents are unaffected (zero behavior change).

app.py

@@ -80,6 +80,9 @@ NODE_LABELS = {
     "handle_undo":        "↩ 撤销操作",
     "handle_reset":       "🔄 重置会话",
     "save_session":       "💾 保存会话",
+    "generate_skeleton": "🏗 生成骨架",
+    "refine_layout":     "📐 精调布局",
+    "map_fields":        "🏷 映射字段",
 }
 
 INTENT_LABELS = {
@@ -133,6 +136,11 @@ def _process_uploaded_file(uploaded_file, suffix: str) -> dict:
         if tt == "full_a4":
             parsed_text = layout["description"]
             parsed_type = "a4_template"
+            # 存储布局 schema 供分层精确生成使用
+            from backend.layout_analyzer import extract_layout_schema
+            schema = extract_layout_schema(layout)
+            st.session_state.agent_state["layout_schema"] = schema
+            st.session_state.agent_state["ocr_elements"] = layout.get("rows", [])
         elif tt == "partial_rows":
             parsed_type = "a4_partial"
             if current_jrxml.strip():
@@ -290,7 +298,8 @@ def run_agent(user_input: str):
                             f"找到 {len(ctx)} 字符参考模板" if ctx else "未匹配到模板"
                         )
 
-                    elif node_name in ("generate", "modify_jrxml", "correct_jrxml"):
+                    elif node_name in ("generate", "modify_jrxml", "correct_jrxml",
+                                       "generate_skeleton", "refine_layout", "map_fields"):
                         jrxml = node_state.get("current_jrxml", "")
                         executed_nodes[-1]["detail"] = f"生成 {len(jrxml)} 字符 JRXML"