docs: correct the telemetry field count and document backpressure

Three README drifts, none of them about issue #8 alone:

- the telemetry row said 18 fields; record_llm_call takes 22 (verified by
  inspect.signature). ports.py claimed 20 in its own docstring, so both
  records of the same fact were stale.
- LLMResponse.cost is hardcoded to None on the chat path (retry.py:519).
  Cost only ever reaches telemetry. The old doc site named this as a known
  trap, so the capability row now says it outright.
- backpressure had no row at all, which is what issue #8 was about.

src/polygateway/ports.py

@@ -245,7 +245,7 @@ class StructuredOutputStrategy(Protocol):
 
 @runtime_checkable
 class TelemetryRecorder(Protocol):
-    """遥测后端;20 字段冻结(M1 设计 §4.4 + issue #3),唯一调用点是 TelemetryEmitter。
+    """遥测后端;22 字段冻结(M1 设计 §4.4 + issue #3/#4),唯一调用点是 TelemetryEmitter。
 
     新增参数不设默认值: 库外无第三方实现者(三项目迁移时删除了各自的同名
     Protocol),完整签名的成本为零,而少写一列会被 emitter 的降级吞成 warning。