feat: add stall_ms and stall_count telemetry columns

Third approved port change: TelemetryRecorder.record_llm_call gains
stall_ms/stall_count (assembly gate derives from this signature).
Schema five edit points (both DDLs, both backfill decls, COLUMNS)
with INTEGER nullable columns appended last; telemetry_schema_sql
regenerates from the same constants untouched. Emitter forwards the
pair from retry._emit on the success branch only (cancelled hedge
loser rows stay NULL by construction); cache-hit and terminal rows
pass explicit None. Both **fields recorders pick the columns up with
zero signature change.

src/polygateway/telemetry/sqlite.py

@@ -143,7 +143,7 @@ class SQLiteRecorder:
                     logger.warning("SQLite 遥测补列失败(写入将逐行降级): {}", exc)
 
     async def record_llm_call(self, **fields: object) -> None:
-        """写一行遥测;字段集合即 36 字段冻结签名(ports.TelemetryRecorder)。
+        """写一行遥测;字段集合即 38 字段冻结签名(ports.TelemetryRecorder)。
 
         取值按 `self._columns`(manual 档可能已被裁剪),与 `self._insert` 的
         占位符同序——两者必须一起改,分开改就是把值写进错位的列。