An address-based build maps each imported registered range through its
host import. UploadPrepare merges a descriptor range or snapshot
overlapping such a range into the range's region, and the region kept
the import even when the merge took it past the import's end, as a V#
crossing into the next registered range does. Descriptor then bound the
import's buffer beyond its size, and the BDA table mapped the whole
region, the next range included, through the import's device address.
A merge that extends a region past its end now drops the import. The
region then takes the import lookup and, with no import covering it,
the copy path, as a range crossing an image mirror's end already does.
Like every copied range, it is read-only to pointer stores.
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