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.
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