fix: emit only registered ARF fields, verified against IANA

Backlog item 4 asked for a primary source for the machine-readable part.
It exists, and it is IANA's MARF registry rather than x-arf's own docs.
Checked against that registry and RFC 5965 itself on 2026-09-10.

Report-Type is not a registered field. The registry lists 25 names and it
is absent; it came from this spec's worked example rather than a decision.
Removed, because Feedback-Type already says it. Keeping it would have been
harmless, since RFC 5965 section 6 makes ignoring an unknown field a MUST
for the receiver, but that same section requires an extension field be
registered, and a desk should be able to look up every field in a document
this tool sends.

Feedback-Type moves from abuse to fraud. RFC 5965 registers fraud as
"indicates some kind of fraud or phishing activity" and abuse as
"unsolicited email or some other kind of email abuse". This tool reports
phishing, and some desks route fraud separately from bulk spam.

Reported-Uri becomes Reported-URI, the spelling in the ABNF and the
registration. The RFC's own worked example uses the other one, which is
where this came from. Names are case-insensitive so nothing was broken.

A new test holds the registry's 25 names and asserts every emitted field
is one of them, with Source named as the single deliberate exception, so
a new unregistered field cannot arrive unnoticed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LByBnw83xr9YP85nskzkyE

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