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

test/services/test_app_service.py

@@ -128,6 +128,33 @@ async def test_chat_sees_set_info_values_and_writes_back_patch():
     assert json.loads(service.get_info("s1", "hphm1")) == "苏B12345"
 
 
+@pytest.mark.asyncio
+async def test_chat_form_patch_mapped_to_business_keys():
+    """图表单键差量经映射写回业务键（作业 6 映射的端到端）。"""
+    service, backend = make_service(form_update={
+        "has_injury": False, "vehicle_count": 2, "accident_cause": "追尾",
+    })
+    await service.chat(_input("s1", "两辆汽车追尾，没有人受伤"))
+    acd = json.loads(service.get_info("s1", "acdinfo"))
+    assert acd["ywrysw"] == "0"
+    assert acd["jdcsl"] == "2"
+    assert acd["sgyy"] == "追尾"
+    # 表单键本身不落 repository（未知业务键 -> JSON 空串）
+    assert json.loads(service.get_info("s1", "has_injury")) == ""
+
+
+@pytest.mark.asyncio
+async def test_set_info_acd_value_mapped_into_form_injection():
+    """set_info 写业务键 -> chat 注入为图表单键（反向映射）。"""
+    service, backend = make_service()
+    service.set_info("s1", "ywrysw", "0")
+    service.set_info("s1", "jdcsl", "2")
+    service.set_info("s1", "bjrjs", "1")   # 无表单来源，不注入
+    await service.chat(_input("s1", "继续办理"))
+    injected = backend.calls[0].injected_fields
+    assert injected["accident_fields"] == {"has_injury": False, "vehicle_count": 2}
+
+
 @pytest.mark.asyncio
 async def test_chat_without_business_state_passes_no_injection():
     service, backend = make_service()
@@ -136,8 +163,9 @@ async def test_chat_without_business_state_passes_no_injection():
 
 
 @pytest.mark.asyncio
-async def test_stream_injects_fields_and_passthrough_events():
-    service, backend = make_service(form_update={"ywrysw": False})
+async def test_stream_injects_fields_writes_back_form_update():
+    """流式：注入同 chat；FormUpdate 事件差量捕获写回后透传。"""
+    service, backend = make_service(form_update={"has_injury": False})
     service.set_info("s1", "xm1", "王五")
     events = [
         event async for event in service.stream(_input("s1", "继续办理"))
@@ -145,6 +173,8 @@ async def test_stream_injects_fields_and_passthrough_events():
     kinds = [type(event).__name__ for event in events]
     assert kinds == ["TextDelta", "FormUpdate", "TextDelta"]
     assert backend.stream_calls[0].injected_fields["person_fields"]["xm1"] == "王五"
+    # FormUpdate 差量已写回 repository（表单键 -> 业务键）
+    assert json.loads(service.get_info("s1", "acdinfo"))["ywrysw"] == "0"
 
 
 # ---- delete：清理 + 幂等 ----