test(signatures): record what the quoted-delimiter test does not pin

The test asserts that a delimiter inside the quoted original is not treated
as this message's signature, and it passes whether or not the code checks
for that. Two mutations were measured against it and both stayed green:
trimming the delimiter comparison so a quoted "> -- " matches, and making
the quoted text one of the known signatures so the match guard could not be
what refuses the removal.

Neither changes the output. blockEnd() stops the block at the quote, so the
quoted signature survives either way, and the behaviour is correct under
both. The comment says so, so the next reader does not spend the same
measurements discovering that the test cannot be sharpened.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEcn3u19xPqv6ggD15PG4c

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