feat: judge whether reasoning actually happened from multiple signals

reasoning_tokens=None has been carrying two meanings at once, no
reasoning and no report, and the library resolved the ambiguity by
quietly claiming the first. ThinkingObservation splits them: UNKNOWN
says the call left no signal, ABSENT says the provider reported zero.

The verdict ranks evidence by hardness. Reasoning prose is the fact
itself; reasoning_tokens is a report about the fact, so a missing
report cannot overrule prose that is right there. The prose check
strips first, since a gateway that returns whitespace is not evidence.

The enum lives in types.py, not in the new thinking.py, because
LLMResponse is typed on it and the innermost layer must not import a
decision module.

src/polygateway/types.py

@@ -10,6 +10,7 @@ import math
 import re
 from collections.abc import Mapping
 from dataclasses import dataclass, field
+from enum import StrEnum
 from types import MappingProxyType
 from typing import Any
 
@@ -166,6 +167,27 @@ def canonical_sampling_json(merged: Mapping[str, Any]) -> str | None:
     return json.dumps(dict(merged), sort_keys=True, ensure_ascii=False)
 
 
+class ThinkingObservation(StrEnum):
+    """一次调用中"推理是否真的发生"的裁定结果(issue #16/#17)。
+
+    三态**不可折叠为布尔**: `UNKNOWN` 是"本次无任何信号,判不出来",与
+    `ABSENT`("上游明确上报了未推理")语义不同。把前者折叠进后者,正是
+    `reasoning_tokens=None` 制造的那个歧义——库据此静默宣称"没推理",而实际
+    可能推理了且已计费(MiniMax-M3 非流式实测: completion 53 vs 关闭档 3,
+    推理正文与 usage 明细双双不回传)。
+
+    裁定由 `thinking.observe_thinking` 做,本类只是取值域。**枚举定义在最内层
+    而非决策层**: 它是 `LLMResponse` 的字段类型,放进 `thinking.py` 会让
+    `types.py` 反向 import 决策模块(P7 依赖铁律)。
+
+    取值进遥测落库,改名即造成历史数据断层。
+    """
+
+    OBSERVED = "observed"
+    ABSENT = "absent"
+    UNKNOWN = "unknown"
+
+
 @dataclass(frozen=True)
 class LLMResponse:
     """一次治理调用的统一响应(与三项目超集兼容,ARCH §5.1)。"""