fix: allowed missing `dmarc.pct` which defaults to 100

[andylizi]

According to RFC 7489, pct is optional and defaults to 100. Current code reject DMARC records without pct=100, which is unnecessarily strict.

For example, gmail.com doesn't set pct=100 for its DMARC. Cloudflare's DMARC generator also omit it by default.

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