feat(services): assignment 6 full - form/business key mapping and stream write-back

- fields_registry: FORM_TO_BUSINESS_KEY bidirectional mapping between
  the graph's six form keys (has_injury...) and the acd business keys
  (ywrysw...); bjrjs/ywmtc have no form source (set_info only);
  business_patch_from_form maps graph diffs to whitelisted business keys
  and filters internal/unknown keys
- app_service: chat injection now maps repository business keys back to
  form keys; both complete() and stream() write back field diffs
  (FormUpdate events captured in stream, closing the skeleton gap);
  form keys never leak into the business repository
- docs/assignment6-compat.md: mapping table documented, stream write-back
  backlog item resolved
- tests: mapping coverage, write-back mapping end-to-end, reverse
  mapping injection (bjrjs/ywmtc excluded), stream FormUpdate write-back

Verified: 205 unit tests

src/services/app_service.py

@@ -13,11 +13,13 @@
 endpoint 只负责校验、协议（SSE / 响应模型）与依赖注入。
 """
 
-from ..backends.chat import ChatBackend, ChatInput, ChatResult
+from ..backends.chat import ChatBackend, ChatInput, ChatResult, FormUpdate
 from .fields_registry import (
     FIELD_REGISTRY,
     GROUP_KEYS,
+    BUSINESS_TO_FORM_KEY,
     FieldGroup,
+    business_patch_from_form,
     parse_field_value,
     to_external_string,
 )
@@ -37,8 +39,12 @@ class AccidentAppService:
     # ---- chat：图每轮读取最新业务状态，产出差量写回 ----
 
     def _injected_fields(self, session_id: str) -> dict:
-        """repository 业务字段 -> 图输入的分组注入（acd 进 accident_fields，
-        两组人员进 person_fields，与 AgentState 的键结构对齐）。"""
+        """repository 业务字段 -> 图输入的分组注入。
+
+        acd 业务键经 BUSINESS_TO_FORM_KEY 映射回图表单键
+        （无表单来源的 bjrjs/ywmtc 不注入）；人员业务键与图的
+        person_fields 同构，原样注入。
+        """
         business = self._repository.load(session_id)
         if not business:
             return {}
@@ -48,11 +54,19 @@ class AccidentAppService:
             if spec is None:
                 continue
             if spec.group is FieldGroup.ACD:
-                injected["accident_fields"][key] = value
+                form_key = BUSINESS_TO_FORM_KEY.get(key)
+                if form_key is not None:
+                    injected["accident_fields"][form_key] = value
             else:
                 injected["person_fields"][key] = value
         return injected
 
+    def _writeback_patch(self, session_id: str, patch: dict) -> None:
+        """图产出的差量（表单键/人员键）经映射落 repository 业务键。"""
+        business_patch = business_patch_from_form(patch)
+        if business_patch:
+            self._repository.apply_update(session_id, business_patch)
+
     async def chat(self, chat_input: ChatInput) -> ChatResult:
         """非流式一轮对话：注入最新业务字段 -> 执行图 -> 写回字段差量。"""
         injected = self._injected_fields(chat_input.session_id)
@@ -65,15 +79,14 @@ class AccidentAppService:
                 injected_fields=injected or None,
             )
         )
-        # 图产出的字段差量（turn_patch / 完整字段）写回 repository，
+        # 图产出的字段差量写回 repository，
         # 使 set/get_info 与后续轮次看到同一事实来源
-        patch = result.form_update if isinstance(result.form_update, dict) else {}
-        if patch:
-            self._repository.apply_update(chat_input.session_id, patch)
+        if isinstance(result.form_update, dict):
+            self._writeback_patch(chat_input.session_id, result.form_update)
         return result
 
     async def stream(self, chat_input: ChatInput):
-        """流式代理：字段注入与写回语义同 chat()，事件流原样透传。"""
+        """流式代理：字段注入同 chat()；FormUpdate 事件捕获写回后透传。"""
         injected = self._injected_fields(chat_input.session_id)
         result_input = ChatInput(
             session_id=chat_input.session_id,
@@ -82,9 +95,9 @@ class AccidentAppService:
             client_mode=chat_input.client_mode,
             injected_fields=injected or None,
         )
-        # 骨架阶段原样透传事件流；字段差量写回在作业 6 完整版随
-        # FormUpdate 事件捕获补齐（对照计划作业 8 的真实增量）。
         async for event in self._backend.stream(result_input):
+            if isinstance(event, FormUpdate) and isinstance(event.data, dict):
+                self._writeback_patch(chat_input.session_id, event.data)
             yield event
 
     # ---- set_info / get_info：零 LLM、零辅助对话 ----