docx: four write-only fields in field_projection.mbt are hidden by declaration-wide #warnings suppressions

[bobzhang]

Surfaced by review during the per-warning-id sweep (PR #496, id [0007] unused_field).

Four fields in docx2html/docx/field_projection.mbt are populated and never read
by production or by the whitebox suites:

• ReaderFieldBoundary.carrier_identity (field_projection.mbt:36)
• ReaderField.separate_marker_identity (:56)
• ReaderField.separate_carrier_identity (:57)
• ReaderField.end_carrier_identity (:60)

They are invisible to moon check because their declarations already carry
#warnings("-unused_field"), added before this sweep. That is the general hazard
with declaration-wide suppression, which MoonBit 0.1.20260821 gives no per-field
alternative to: the attribute covers fields added later, and fields that go dead
later.

Not fixed in #496 because they are not [0007] diagnostics on any backend, and
removing them touches the field-classification construction and assignment sites,
which deserves its own review.

Worth checking while doing it: whether any of the three mutable ones was supposed
to be consulted by a consumer, in which case the finding is a bug rather than
dead weight.