feat(signatures): replace an existing signature, guarded by a match

Finding a "-- " delimiter is not authority to delete what follows it. The
block is replaced only when its text matches one of the signatures on disk,
and otherwise the new one is inserted with nothing removed, so a wrong guess
adds a visible duplicate rather than destroying the user's writing.

A quoted delimiter is never the signature either: the quoted original
carries the other party's, and it is not this message's to replace.

The block's lower boundary is quoteStart(), not the first quoted line. The
attribution introduces the quote and belongs with it, so scanning for '>'
alone swallowed "On Mon, someone wrote:" into the signature block: it then
matched no known signature, and had it matched, removal would have stranded
the attribution above the text it introduces. The boundary the insertion
uses and the boundary the removal uses have to be the same one.

The guard's test was mutation-checked by making the match unconditional,
which fails it.

Part of item 152.

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