F2: the boundary test always used message_id "m-0", so a gate that measures
payload.len() + 231 -- exactly that id's fixed overhead -- passed unnoticed.
message_id rides the encoded envelope and echoes the client's own value, so a
longer id changes the overhead the gate must measure honestly.
Parametrize synthetic/sized_exactly by message_id and add a second boundary
test at a 40-byte id: 1,228,800 bytes still passes and 1,228,801 still
refuses. Add mutation entry D20L6, the "+231" formula, which dies against the
new test and survived the old one.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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