_schedule_polling_recovery promised the gateway 'stays alive and will retry' for every error, but a _PollingStallError goes straight to _go_fatal_network (supervisor rebuild). Branch the wording on the error type, drop the watchdog's own pre-log so a stall yields exactly one error-level line (from _go_fatal_network), and carry stalled_for/generation in the stall error text instead. Test module docstring and test name updated to match the hand-off semantics.
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