Both existing scope-digest forgeries differ in length from a real
`sha256:` + 64-hex digest, so a length-only comparison in
`exact_opportunity` passed every external admission test.
The new test takes two genuinely issued controller opportunities, lets
one subject speak (genesis places every subject in the commons, so the
telling lands as knowledge on the other and moves its scope digest), then
submits the listener's now-stale opportunity. It asserts the stale and
current digests differ and share a length, and that the submission is
refused with ScopeChanged and commits nothing.
A stale SHA-256 digest differs from the current one almost everywhere, so
it cannot catch a comparison that skips the last character. The test also
submits the current digest with only its final hex digit changed.
Mutants on the comparison, each killed: length-only, first seven
characters only, all but the last character.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
bde05de4e3