The stray-sealing regression test asserted copied < 2 MiB — about 4x the
defended sealed shape, so reverting the seal to a re-merge (or removing it)
left the test green. Measured both shapes as the fd-3 sibling does: the sealed
shape copies ~120 KB, the re-merge shape ~538 KB. Tighten the bound to 256 KiB,
which sits between them, and record the measurements in the comment and the
Agent Note so the fail-before claim holds.
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