fix: address completion verdict review findings

- fidelity cacheability also requires a non-empty projected body, so
  think-tag-only answers are no longer cached
- a no-argument tool sent with arguments null only is a deterministic
  dialect: ResultInvalid tool_arguments_null instead of retried
- text mode treats choice index null as 0 and no longer lets False pose
  as 0
- null-only tool_calls elements no longer produce an empty tool_calls
  list; non-stream fidelity checks role before the verdict; warning
  paths are length-bounded
- docs: ARCHITECTURE 7.8 field counts, README cache/watchdog wording,
  CHANGELOG notes, review record in the design, hook note in the plan

Red: 8 new tests failed before this change (e.g.
test_openai_cacheable_requires_projected_body,
test_null_arguments_tool_is_result_invalid_stream,
test_text_mode_choice_index_null_is_zero[None-True]).

tests/unit/test_completion_transport.py

@@ -85,7 +85,7 @@ async def test_eof_after_finish_usage_only_frame_and_tools():
     result = await _run(frames, done=False, mode="openai")
     tool = result.completion.message["tool_calls"][0]
     assert tool["function"] == {"name": "lookup", "arguments": '{"q": 1}'}
-    assert result.stream_end == "eof_after_finish"
+    assert result.stream_end == "eof_after_finish" and result.usage_source == "measured"
 
 
 @pytest.mark.parametrize(
@@ -416,3 +416,38 @@ async def test_openai_reasoning_length_single_attempt_via_client():
         await client._transport.aclose()
     assert len(calls) == 1 and response.call_stats.attempts == 1
     assert response.stream_end == "eof_after_finish" and response.finish_reason == "length"
+
+
+async def test_eof_after_finish_stop_single_attempt_via_client():
+    """#31 验收 1 经治理链: stop + usage 无 DONE,attempts=1、不重试到耗尽。"""
+    from tests.unit.test_client import _client
+
+    calls = []
+
+    def handler(_):
+        calls.append(1)
+        return _sse_stream(_assistant(content="ok"), _f({}, "stop", _USAGE), done=False)
+
+    client = _client(handler=handler)
+    try:
+        response = await client.chat([{"role": "user", "content": "hi"}])
+    finally:
+        await client._transport.aclose()
+    assert len(calls) == 1 and response.call_stats.attempts == 1
+    assert response.stream_end == "eof_after_finish" and response.usage_source == "measured"
+
+
+async def test_text_salvage_with_length_partial_content_delivered():
+    """文本 salvage + length + 部分正文(finish 后无 usage): 交付、salvaged。"""
+    frames = [_assistant(content="part"), _f({}, finish="length")]
+    result = await _run(frames, done=False, missing_done="salvage")
+    assert result.content == "part" and result.stream_end == "salvaged"
+    assert result.finish_reason == "length"
+
+
+async def test_null_arguments_tool_is_result_invalid_stream():
+    """审查 I2: 无参函数 arguments 全程 null → ResultInvalid,不重试。"""
+    tool = {"index": 0, "id": "c", "type": "function", "function": {"name": "f", "arguments": None}}
+    frames = [_assistant(tool_calls=[tool]), _f({}, "tool_calls", _USAGE)]
+    with pytest.raises(ResultInvalidError, match="tool_arguments_null"):
+        await _run(frames, done=True, mode="openai")