docs: record the real-mail sweep and open a backlog

A synthetic fixture only ever holds the shapes someone thought of, and
the shapes nobody thought of are the ones that leak. Sweeping the user's
own tag:spam is the only source of those, and it earned its place: the
first message found a header the parser did not read, and the corpus
exercised a spoofed Reply-To display name no fixture had.

AGENTS.md records it under Testing, beside the fixture rule it is the
deliberate exception to: ask first, work from the scratchpad, compare
every address in the raw source against the whole ioc output, and let
only counts and stripped domains out. A finding becomes a synthetic
fixture; the real message never enters the repository.

BACKLOG.md opens with the one thing the sweep found that was not worth
fixing blind: boilerplate namespace urls (w3.org doctypes) reported as
indicators. Noise rather than a defect, and the argument for leaving it
alone is real, so it is written down with both sides rather than built.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019NHaqA1Rz5ybed7wFUeQbK

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