Pins the ARCHITECTURE section 6.1 revision to land before or with the
implementation, since the new scope reason contradicts the current single
source of truth. Documents why SourceNotConfiguredError may sit outside
the four-way classification: that rule governs transport-translated call
failures, and the GatewayUnavailableError family already lives outside it.
Also collapses the ten per-field response ternaries in emit_attempt into
an _AttemptUsage view. They all expressed the same decision and pushed the
method to cyclomatic complexity C, which blocked the commit gate.
Fail-closed governance backend failures are semantically scope-level
unavailability, yet GovernanceBackendError sits directly under
PolyGatewayError, so callers writing only `except GatewayUnavailableError`
drop them into the catch-all bucket and burn their failure budget on a
fault that a restart would clear.
The design reparents it under GatewayUnavailableError with a new
governance_backend_down reason, splits the two "unknown source" sites into
a separate assembly-defect error so a misconfiguration still reaches the
dead letter queue, and picks a non-zero retry_after_s to avoid a
zero-delay retry storm against a backend that is already down.
iomgaa