fix(compose): correct the header's attribution and harden three tests, item 123

The header still credited CMARK_OPT_SAFE after the .cpp comment and the test
were corrected, which left the wrong mechanism named in the file
MessageBuilder's author will actually read.

Three test weaknesses, each measured rather than assumed. The accented-text
test survived a SYMMETRIC latin-1 mutation, since the round trip cancels for
codepoints under U+0100, so it now carries a character latin-1 cannot
represent. The tasklist test asserted on the bare word "checked", which
ordinary prose would satisfy, and now asserts the attribute. And the
extension registration is wrapped in a function-local static: cmark-gfm's
registry has no once-guard, and this project has a worker thread, so the
first call racing itself would tear the registry rather than crash cleanly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015muoUo2GdxmBDSp5vjYcbE

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