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_fields_registry.py

@@ -6,8 +6,11 @@ from src.services.fields_registry import (
     FIELD_REGISTRY,
     GROUP_KEYS,
     INTERNAL_FIELD_KEYS,
+    FORM_TO_BUSINESS_KEY,
+    BUSINESS_TO_FORM_KEY,
     FieldGroup,
     SensitiveLevel,
+    business_patch_from_form,
     parse_field_value,
     to_external_string,
 )
@@ -87,3 +90,32 @@ def test_internal_field_protection_listed():
     """图内部键登记在保护清单中，set_info 白名单不会收录它们。"""
     for key in INTERNAL_FIELD_KEYS:
         assert key not in FIELD_REGISTRY
+
+
+def test_form_business_key_mapping_covers_form_fields():
+    """图表单六键全部映射到 acd 业务键（作业 6 建立的映射）。"""
+    assert set(FORM_TO_BUSINESS_KEY) == {
+        "has_injury", "has_non_motor_vehicle_hit", "is_on_scene",
+        "vehicle_count", "accident_time", "accident_cause",
+    }
+    for business_key in FORM_TO_BUSINESS_KEY.values():
+        assert business_key in FIELD_REGISTRY
+        assert FIELD_REGISTRY[business_key].group is FieldGroup.ACD
+
+
+def test_business_patch_from_form_maps_and_filters():
+    """表单键映射、人员键原样、未知键过滤。"""
+    patch = business_patch_from_form({
+        "has_injury": False,           # -> ywrysw
+        "vehicle_count": 2,            # -> jdcsl
+        "xm2": "李四",                  # 人员键原样
+        "stage_code": "1002",          # 内部键过滤
+        "no_such_form_key": "x",       # 未知键过滤
+    })
+    assert patch == {"ywrysw": False, "jdcsl": 2, "xm2": "李四"}
+
+
+def test_business_keys_without_form_source_not_injected():
+    """bjrjs/ywmtc 无表单来源：不进入注入映射（只能 set_info 直写）。"""
+    assert "bjrjs" not in BUSINESS_TO_FORM_KEY
+    assert "ywmtc" not in BUSINESS_TO_FORM_KEY