Adjust binance stale-bar detection to 2x tolerance

Change the stale-bar check in `.binance.feed` from `timeframe` to
`timeframe * 2` tolerance to avoid false-positive pauses when bars
are slightly delayed but still within acceptable bounds.

Styling,
- add walrus operator to capture `_time_step` for debugger
  inspection.
- add comment explaining the debug purpose of this check.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/brokers/binance/feed.py

@@ -275,12 +275,14 @@ async def open_history_client(
                     f'{times}'
                 )
 
+            # XXX, debug any case where the latest 1m bar we get is
+            # already another "sample's-step-old"..
             if end_dt is None:
                 inow: int = round(time.time())
                 if (
-                    (inow - times[-1])
+                    _time_step := (inow - times[-1])
                     >
-                    timeframe
+                    timeframe * 2
                 ):
                     await tractor.pause()