GovernanceBackendError arrived with the M2 distributed backends but never
made it into the section 6.1 table, so it had no place in the taxonomy
callers actually read. That omission is why the README missed it too.
Records the reparenting, the new governance_backend_down reason, and why
SourceNotConfiguredError deliberately stays outside the reparented family:
a misconfigured source name must burn its failure budget and surface,
not retry forever in silence.
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