README sanctions retiring datatypes.rs ahead of M4. The module was a
pure relay (resync boundary -> datatypes -> mod.rs facade) with no
external consumer importing it directly, so the facade now re-exports
ResyncStatusType from replication_resync_boundary and the relay file is
deleted. Consumers stay behind the ECStore facade, keeping Migration
Rule #15 intact — the original retirement wording ("consumers import
through rustfs-replication directly") conflicted with that rule and is
corrected in the README.
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