Replace `_looks_like_polling_stall` (a three-substring classifier over
`str(error)`) with `_PollingStallError(RuntimeError)`. The watchdog and the
post-reconnect verifier raise it at their two stall sites; the reconnect
ladder checks `isinstance` through `_iter_exception_graph` before handing
the adapter to the supervisor.
WHY: a substring match couples recovery routing to log wording (rename a
message, silently lose the fatal handoff) and can misfire on unrelated
errors that quote the same words. A typed exception keeps #113657's
behaviour with no behavioural change beyond the classifier's source.
cb4063f76a