Section 6.1 already prescribed the monotonic group_id anti-replay guard;
add the companion optional freshness check — a verifier MAY bound the
signed group_end against its own clock for the group it treats as the
current tip, never for historical resync (whose timestamps are legitimately
old). Matches the reference verifier's opt-in checks.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
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