README gains the key with the reason a single-source scope wants wait,
and the price of choosing it. .env.example carries the same warning
since README points at it as the full key list. ARCHITECTURE 7.4 records
why the missing cell is unrelated to source count -- and why keying on
len(sources) would be the worse debt -- plus the six-exit retry_after_s
contract and the admission convergence; 9 registers the key.
CHANGELOG stays unreleased per the release checklist: the version bump
belongs to the release run, not here. Its "read this first" section
covers the half-open retry_after_s change, which is visible even on the
default fail_fast setting.
iomgaa
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