feat: add a content event and stall accounting to the transport port

T1(计划 §4):观测原语与端口契约(issue #27/#28 底层)。

- Transport.complete 加必填 kw content_event(顺序在 first_token_event
  后,无默认值,同款约定):流式每增量 chunk 置位一次,非流式/对冲路
  恒不置位,None = 不观测
- TransportResult 增 stall_ms/stall_count(带默认 None):超阈值相邻
  增量 chunk 沉默总额与段数,chunk<2/非流式 → None,尾部不计
- SourceConfig 增 stall_threshold_ms(默认 1000,正整数拒 bool,
  _validate_watchdog 同款落点)
- _complete_stream 记增量 chunk 时间戳(空 chunk/usage 帧/keepalive
  不产生记录点)+ 模块级纯函数 _stall_from_chunk_times(严格大于
  阈值才减);max_inter_token_ms 原语义一字不动
- §2.2 同步清单全量:11 实现 + 11 调用点(重跑 grep 复核零遗漏)
- 批次 A 12 用例先红(TypeError/ImportError/AttributeError)后绿,
  证据 tests/outputs/138/t1/

tests/unit/test_ports.py

@@ -73,7 +73,16 @@ class _DummyMw:
 
 class _DummyTransport:
     async def complete(
-        self, *, messages, source, stream, overlay, call_id, reasoning_effort, first_token_event
+        self,
+        *,
+        messages,
+        source,
+        stream,
+        overlay,
+        call_id,
+        reasoning_effort,
+        first_token_event,
+        content_event,
     ):
         raise NotImplementedError