docs(specs): guard the signature splice against deleting the user's text

Finding a "-- " delimiter is not enough to authorise removing what
follows it. The block is replaced only when its text matches one of the
signatures on disk; otherwise the new signature is inserted and nothing
is removed.

"-- " can reach the buffer without the user ever choosing a signature,
most plausibly pasted in with quoted text from another client, and the
unguarded scan would have deleted everything after it silently. The
failure is now directional: a block that matches is replaced, and one
that does not produces a second signature, visible in the editor and one
undo away. A wrong guess adds text rather than losing it.

Two markers were considered for the same problem and refused, both
recorded with the reasons. A zero-width character ships in the sent
message, fingerprinting the client in outgoing mail, and has to survive
the draft round trip through GMime, quoted-printable and MimeParser,
which is the pipeline that normalises such characters away. A doubled
delimiter is not the RFC 3676 separator, so no receiving client would
recognise the signature, and it would not have caught the pasted-text
case that prompted it.

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

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