Ports the SkipPdfNormalization setting from Verify.PDFium (3fd22e9).

Adds a SettingsTask/VerifySettings extension backed by a context key, so a caller
whose producer already emits byte-deterministic documents can snapshot the bytes
as produced. Normalizing those again copies the buffer, rescans it, and — when
the XMP packet is canonicalized — rebuilds it and repairs the cross-reference
table, all to change nothing.

Default behaviour is unchanged: without the setting the bytes are normalized.

SkipPdfNormalizationTests asserts the pairing rather than just that the setting
round-trips: the skipped snapshot holds the producer's own bytes and the default
one holds the neutralized bytes. A premise test pins that the sample is not
already normalization-invariant, so the pair cannot both pass vacuously.