Gate finding (quality, Low): _IngressDispatchStallError had no consumer -
every branch checks isinstance(error, _PollingStallError) - so the subclass
and its docstring were dead weight. One stall was also announced three times
(the "healthy but deaf" pre-log, the handoff warning, the fatal error line);
the sibling _check_polling_stall deliberately skips a pre-log. Drop the
subclass, drop the pre-log and carry the "healthy but deaf" counts in the
error text instead, and read the polling generation once.
Tests assert on the error handed to the stubbed handoff rather than caplog.
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